In today's competitive business world, performance audits play a big role in learning and continuous improving UX. As I had quite some time with performance auditing, I learned invaluable lessons from both successes and missteps, how to estimate and apply my findings. Auditing is a creative process with unexpected findings and learnings that makes the work on web performance so interesting and fulfilling. In a conventional audit document this is perspective is hidden completely. The following document is a unconventional mix of an audit document, tips to structure data and text as well as written thought processes, offering you a methodology. By including all of my miss assumptions and fails, it is a more natural way of learning then reading a conventional audit. I hope it helps you to see the creative process and fun behind web performance auditing! And maybe it encourages you to ensure good CWV, UX and energy efficiency for your company.

Michael Hladky


Comparison - Filmstrip

Comparison - Filmstrip -- Observable HQ Performance Audit Example

FP/LCP Hero Video /Idle



LCP - Hero Video










Comparison - Flame charts

Re-apply DOM

Re-apply DOM -- Observable HQ - Performance Audit Example

Recalculate styles

Recalculate styles -- Observable HQ - Performance Audit Example

Scroll down and up again

Scroll down and up again -- Observable HQ - Performance Audit


Idle -- Observable HQ - Performance Audit

Average Improvements in milliseconds


Ø Task before

TTB before

Ø Task after

TTB after


Re-apply DOM

110ms / 160ms

90ms / 110ms

27ms / 21ms


numbers apply to add DOM task/ recalculate styles task

Recalculate styles





numbers apply to recalculate tasks

Scroll down/up





taksk of bissy sections








Use the script in webpagetest or as DevTools snippet to reproduce the changes. To do so just copy the raw content from here.

Show me how to reproduce it quickly

To quickly check it in your browser open the DevTools "Sources" tab and click on "Snippets". Create a snippet with the code above, execute it and measure the impact.

Snippet with the code -- Observable HQ - Performance Audit Example

DevTools "Sources"

  1. Open DevTools

  2. Select Sources Tab

  3. Select Snippets

  4. Click New snippet

  5. Give it a name

  6. Insert scripts

  7. Execute

  8. Check console


  1. Open webpagetest

  2. Select "Site Performance" as test

  3. Select "Advanced"

  4. Paste the optimizations in the sectin "Inject Script"

  5. Click on tab "Block"

  6. Paste annie-avatar.jpg ramona-avatar.jpg in the section "Block Requests Containing"

  7. Run test

  8. Check result

Main Page

Observable HQ - Main Page Desktop --Observable HQ - Performance Audit

The pages, from the first glimpse, contain a lot of images and I guess live demos of the editor. It took quite a while to load the LCP video, but maybe it's because I sit in the kitchen instead of at my desk. 🙃 There, I always have bad connection. 💤

To get a good first overview let's start with the visible part in more detail.

Page Elements

Main Page Areas -- Observable HQ - Performance Audit Example


  • tool-bar - document.querySelector('')

  • section - document.querySelectorAll('#__next > .jsx-2b91b8133a45a7a2 > .jsx-2b91b8133a45a7a2')

    • hero-section - document.querySelector('#__next > .jsx-2b91b8133a45a7a2 > .jsx-2b91b8133a45a7a2')

      • avatars - document.querySelectorAll('.marketing-presence-widget.jsx-140043cc736fed23:nth-child(1)') or :nth-child(2) for second avatar

      • video (LCP) - document.querySelectorAll('.mw-section video')

    • carousel - document.querySelectorAll('.jsx-1511261573 > .jsx-1511261573 > .jsx-1511261573')

    • DOM animation - document.querySelector('.jsx-6e9c885e3fde48d5')

  • footer - document.querySelector('footer')

Tip: The areas are collected through my audit. Whenever I was identifying an element I was also saving the selector here. I included the method e.g. querySelector to get the list of all relevant elements with the snippet. This will save me some time and I don't need to maintain it in DevTool yet.

document.querySelector('footer') -- Observable HQ - Performance Audit Exampledocument.querySelector('footer') -- Observable HQ - Performance Audit Example

Initial state

To start with our audit we need a reproducible way to measure the status quo, and in what conditions we took the measure.

Audit setup

Target I took the Observable HQ site for the audit as it is a really nice tool and I could maybe provide some benefit to them.


Conditions I used "native throttling" in this audit 🤣, which means all my measures are done in my kitchen with flaky internet connection and no power plug in. 😅 The only thing I made sure was to have everything open in incognito mode.

Device All the measures are done on a ThinkPad X1 (i7 8th Gen) on a Windows OS

Base Measures

The following measures were taken:

  • Page refresh (a full page refresh triggeres over the performace panel)

  • Page re-dom (a full removal and adding back of the DOM)

  • Page re-calculate (transformZ 0.1 on body)

  • Page idle (after page is fully loaded some seconds of no interaction)

Measure Process

All measures are produced with small scripts I hold in my DevTools -> Snippets tab.

This is how I performed the measures:

Measure Process -- Observable HQ - Performance Audit Example
  1. Open the Performance tab

  2. In the bottom section, select the Quick source tab (and close all other tabs)

  3. Start profiling with Ctrl + E or the ⏺ button

  4. Click on the ▶ button to execute the script

  5. Start profiling with Ctrl + E or the ⏹ button

Tip: The console tab pops in after every script execution. This is annoying because we have to click on the Quick source tab again. To lessen the pain and live-hack it we can shrink the output area to the minimum. By doing this we can have a short distance between the tabs and the execute button.

Filmstrip Pageload unoptimized

Filmstrip Pageload unoptimized -- Observable HQ - Performance Audit Example

Page Refresh unoptimized

Page Refresh unoptimized -- Observable HQ - Performance Audit Example

Page Re-draw DOM unoptimized

Page Re-draw DOM unoptimized -- Observable HQ - Performance Audit Example

To reproduce the measure:

  1. Setup the script below as snippet in DevTools -> Sources -> Snippets

  2. Setup DevTools as described in "Measure Process" above.

  3. Start recording

  4. Execute script over "Quick source"

  5. Stop recording

  6. Analyse flames and save the profile if needed


Page Recalculate unoptimized

Page Recalculate unoptimized -- Observable HQ - Performance Audit Example

To reproduce the measure:

  1. Setup the script below as snippet in DevTools -> Sources -> Snippets

  2. Setup tedTools as described in "Measure Process" above.

  3. Start recording

  4. Execute script over "Quick source"

  5. Stop recording

  6. Analyse flames and save the profile if needed


Page Scroll

Page Scroll -- Observable HQ - Performance Audit Example

To reproduce the measure:

  1. Setup the script below as snippet in DevTools -> Sources -> Snippets

  2. Setup tedTools as described in "Measure Process" above.

  3. Start recording

  4. Execute script over "Quick source"

  5. Stop recording

  6. Analyse flames and save the profile if needed

Page Idle

Page Idle -- Observable HQ - Performance Audit Example

To reproduce the measure just record the page without any interaction for some time.


High-Speed Web Applications

JavaScript Performance Beyond the Basics

Audit Documentation

In the following I will document the different phases of the audit including my thought processes.

First Pokes

After my first impression of the flames and the fact that I can't touch code nor assets I decided to focus first on the things I can easily test and measure. This includes runtime measures of DOM and CSS changes.

Here the transferred list from my handwriting as I was too lazy to start a readme right away.

First Quick Findings


  • script driven animations

  • bad lib for DOM animation

DOM & Styles:

  • animation noise

  • styled components triggering refetch of resources (check in hero section)


  • no UI facade for hero GIF

  • lazy loading options present

  • no compression

  • wrong dimensions

To be more productive I try to focus the audit process on the same technique across the page and then switch to the next one I think is applicable.

Phase 1 - Low hanging fruits & discovery


After a look in the delivered HTML I have a little hope we can land an impact with lazy loading resources as they already have it in use pretty much every where.

Let's quickly check the images without loading lazy on the page...

82 of 137 imgs eager (LCP included)

Let's give it a quick try 😁

At pageload 31 images are loaded, after all images are loaded lazy 13 are loaded.

contain and content-visibility

I start with the quick wins first and work my way through the previously defined sections.


Tool-bar -- Observable HQ - Performance Audit Example

The tool-bar is my first candidate. A clear case for contain:strict as (even if itself is without fixed dimensions, its first and only child is) we can assume static dimensiones. I did a check for mobile, to be sure it is the same there, which was the case: height: 55px.

It will be off screen when we scroll so we can consider content-visibility and contain-intrinsic-size.

I don't measure as I don't assume any big impact.

The interactions with tool-bar elements did not show any animated changes nor dropdowns. The only interesting thing was that when I clicked the searchbox a full page overlay showed up. At the beginning I did not see it but after some interaction I spotted a flicker in the tiny images of the headline.

toolbar -- Observable HQ - Performance Audit Example

Let's make a note for the hero section to analyze this.


The majority of the pages content is organized in sections with mid size DOM size. In general the site is media heavy but there are some specific sections containing more relevant animation or media we could have a look at.

We can try if their content is stay stable if we apply content-visibility:auto... It is quite a hick up in the scrollbar, an intrinsic size of 300px makes it way better. Now a quick check on mobile... It's, at least with touchpad and laptop noticable that the scroll is a bit janky, but lets keep it for now and take a measure.

Looks good! Recalculate styles and redom shows pretty nice improvements already.

Section - Examples

One of the interesting sections is the examples section. There we have 2 carousels containing main cards with images.

Their position is animated with translateX which is already pretty good. As a side effect the paint area is huge.

Section - Examples -- Observable HQ - Performance Audit Example

Here we can try to apply contain and content-visibility. Especially content-visibility will have the impact.

As this most probably will have an impact on other images and paint heavy assets too, let's add another rule for all img and video tags:

After we applied the styles we can see in the layers panel that the paint area is now limited to the cards visible in the viewport or obscured. Another check in the layers panel shows us the affected nodes.

Section - Examples 2 -- Observable HQ - Performance Audit Example

Section - Usage

A quick look with the paintflash feature shows that again they did quite a good job, transition is used to run the dimensional changes.

Section - Usage -- Observable HQ - Performance Audit Example

Maybe a small improvement could be done with will-change? I have to understand the animation first...

will-change -- Observable HQ - Performance Audit Example

From what I understand now, the animation is driven by transform and some properties are translated. The animated elements are all contained by on container with fixed width and height. Some elements are animated out of the container border-box and faded out.

I can access all selected elements like this document.querySelectorAll('.jsx-6e9c885e3fde48d5 > div').

I can start now to fiddle around, but first I have to stop all the background noise so I can focus on the one animation.

As the DOM changes and it's hard to make changes directly on the element in the Elements tab, I first add a class that I can target:

Ok, unfortunately my default approach does not work, the class attribute is controlled by JavaScript and I have to go with a data attribute:

With a little bit of CSS I can now target and visually identify the involved elements:

After some time spent with those 6 elements I did the following thing:

I could not measure any impact clearly so I move on 🤷‍. Nevertheless, I have now 2 snippets to remove noise from the page. 🏆


The footer is another clear candidate for our silver bullets ;), Let's see what we can do.

Pretty nice we could even maintain the height exactly.

Phase 2 - View Port and LCP Detailed Look

Section - Hero

The hero section maintains a little bit of fancy text and a video.

From the toolbar review I have a note regards a flicker in the tiny images of the headline on the right. When opening and closing the search overlay I realized that some images are constantly loaded. 2 images visible in the small bubbles in the headline.

Section - Hero -- Observable HQ - Performance Audit Example

A second look in comparison to the rest of the resources showed that these 2 images are with far distance the biggest on the page. 🤣 Due to the usage of CSS and the background-image attribute the priority is always high so there is no chance our LCP content gets loaded first.

Size and Priority -- Observable HQ - Performance Audit Example

I assume it is triggered by react's CD and the usage of CSS variables as background image but I'm not sure ATM.

For now I will keep it with a note to research later...

LCP Asset

The video tag on the right is streamed so the first image can get displayes early on.

Here we can apply our finding from above to our advantage. We can use an image and set it as the background image of the video tag. As this is our LCP and the CSS rule will fetch it with high priority we could create an optimized image for the first paint to improve LCP.

In the snippet below I just used an image from the cards to showcase the effect. For a visual feedback you can add a background color to check the impact visually.

Phase 3 - Hero section avatar images

I'm back at the image flicker 🥰. This was gripping my attention from the beginning but I was not sure if it has enough potential to dig in deeper, so early on. Now that I am pretty satisfied with my first findings I can finally have a closer look here.

I realized I did not put the selector for the small bubbles containing the avatar image in the doc before... 🙄 Again a turn with the element inspector ... meh! ... and the console.

Here we go:

Now as I remember, I wanted to use a random profile picture from a card to show case the impact. Let me do that right now where I am in the elements panel mood...

Ok. Now as I am prepared with some snippets let's finally dig.

I will open the network tab to see if a block of those image URL's has an impact.

Snippets -- Observable HQ - Performance Audit ExampleRequests and Avatar blocked -- Observable HQ - Performance Audit Example

Woof, only from observing it over the screen it is a drastic difference. Also the network waterfall looks way more "parallel".

It definitely pays off to think about a solution here!

My first try to change "something" was to just replace the CSS variable value, but it seems it is somewhere recalculated to the same value and set again... hmm.

Let's try to go with static CSS values and selectors.

Voila! 💪 The refetching is now gone and also a properly sized image is used, only the fetch priority is still on High as we use CSS background-image. But you know, Rome... As a last improvement I will edit the DOM structure a bit to get native lazy-loading and priority in place.

Perfect after this improvement they also have fetch priority low 🥳🥳🥳. Bam... I would say let's call it a day. 🌑 The snippets to re-measure are now pretty nice to use and I'm excited to run the measures with the improvements and compare them after some 😴.

Phase 4 - Hero section video

After I added the optimized measures and the TL;DR section I tried to somehow visualize the hero video impact as it is a major improvement of this audit and therefore it should shine.

Let's use filmstrips to visualize the impact:


Before -- Observable HQ - Performance Audit Example

I started by fixing the initial height of the image with min-height:427px to get less visual changes measures and a better UX.

Placeholder polishing

To make the smithc from the placeholder to the video less visible I create a custom placeholder image. I make a screenshot, open upload my image and start tweaking until I have around 9KB (initially it was 105KB) and used to make it a base64 string.

I started to preload it to get a faster paint and after some measures I decided to inline it. Way faster and easier to apply. The background-image: url() is used here.


To improve the video loading I started to experiment with preloading attributes on the image, but as this will still wait to fetch until the DOM is fully parsed I went with <link rel="preload" as="fetch" href="">.

I'm nervous... :)

Example -- Observable HQ - Performance Audit Example


This nice improvement series deserves a image showing the evolution of this improvement.

Improvements -- Observable HQ - Performance Audit Example

Herosection final snippet

🗣 Memo to myself: Never maintin base64 images in a readme again!

Optimized State

Measures after application of all optimizations

Filmstrip Pageload optimized

Filmstrip Pageload optimized -- Observable HQ - Performance Audit eXAMPLE

Page Refresh optimized

We can't run this comparison easily so we skip it for now.

Page Re-draw DOM optimized -- Observable HQ - Performance Audit  Example

25ms before 154ms

Page Recalculate optimized

Page Recalculate optimized -- Observable HQ - Performance Audit Example

15.45ms before 194ms

Page Scroll optimized

Page Scroll optimized -- Observable HQ - Performance Audit Example

No busy areas anymore.

Page Idle optimized -- Observable HQ - Performance Audit Example

5ms before 20ms


Measurements in comparison

Filmstrip Pageload comparison

Filmstrip Pageload comparison -- Observable HQ - Performance Audit Example

Page Re-draw DOM comparison

Page Re-draw DOM comparison -- Observable HQ - Performance Audit Example

Page Recalculate comparison

Page Recalculate comparison -- Observable HQ - Performance Audit Example

Page Idle comparison

Page Idle comparison -- Observable HQ - Performance Audit Example



Raw files and screenshots of measurements can be found in the /raw directory.

Performance profiles

Raw performance profiles can be found in the /raw directory and viewed in GitHub or in the performance view locally or online.






24.16 MB



8.6 MB



13.37 MB



21.61 MB



20.48 MB



16.53 MB



14.62 MB



10.81 MB