#### PerformanceNewsletter
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#### Issue 53
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#### Editor’s Note
It can become incredibly easy to glance over data and find what we believe, is a reason for the change. It can also be easy to find confirmations in data if we are already set on a particular belief or conviction.
Confirmation bias can lead us astray in life and, consequently, performance work. Approaching data with diligence, curiosity and open mind is crucial. Navigating the thin line between what we *think* is real and what our performance knowledge points us to is ambiguous and sometimes contradictory.
Remember that correlation does not imply causation.
— Karolina
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#### article
## Understand why we need better metrics for a11y
In his article, [Scott Jehl](https://twitter.com/scottjehl) points out significant accessibility shortcomings of performance metrics. With the help of accessibility experts, such as [Léonie Watson](https://twitter.com/LeonieWatson), he calls for the creation of new metrics that help include and portray performance when using assistive technologies.
[]()[We need more inclusive web performance metrics →](https://www.filamentgroup.com/lab/a11y-ready/)
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#### tools
## See speed of sites using different frameworks
Perf Track leverages several tools, such as the HTTP Archive, CrUX, WebPageTest and Lighthouse, to provide benchmarks of sites using selected JavaScript frameworks. See the spread of Web Vitals and other critical measurements based on real data. 👏🏻 [Houssein Djirdeh](https://twitter.com/hdjirdeh).
[]()[Perf Track: Tracking framework performance at scale →](https://perf-track.web.app/)
#### article
## Know if using prefetch is degrading speed
[Andy Davies](https://twitter.com/AndyDavies) explains how using a CDN with not ideal HTTP/2 resource prioritisation in combination with prefetch can have a detrimental, instead of a positive effect on performance. Turns out, in some cases, prefetch might cause more trouble than gain.
[]()[Rel=prefetch and the Importance of Effective HTTP/2 Prioritisation →](https://andydavies.me/blog/2020/07/08/rel-equals-prefetch-and-the-importance-of-effective-http-slash-2-prioritisation/)
#### article
## Analyse which image format performs best
[Johannes Siipola](https://twitter.com/JohannesSiipola) analyses data to see if WebP is really as small as it’s believed to be. In the process, he showcases how exciting the future release of the AVIF format will be, introducing even more size and bandwidth savings in comparison to its predecessors.
[]()[Is WebP really better than JPEG? →](https://siipo.la/blog/is-webp-really-better-than-jpeg)
#### article
## Use custom timings to fix front-end performance
[Richie McColl](https://www.linkedin.com/in/richie-mccoll-3b0ab498/?originalSubdomain=uk) explains how to use the User Timings API to uncover performance of specific interactions in the front-end. With the help of Chrome DevTools, you will be able to measure, analyse and fix the speed of most critical actions.
[]()[Visualising Front-End Performance Bottlenecks →](https://medium.com/dazn-tech/visualising-front-end-performance-bottlenecks-4a0563407679)
#### video
## Explore and reduce the amount of your CSS
[Jhey Tompkins](https://twitter.com/jh3yy) shows a new, experimental feature of Chrome DevTools. With CSS Overview, you will be able to get high-level information about your CSS and identify unused styles.
[]()[Explore a Site’s CSS with Chrome DevTools CSS Overview →](https://egghead.io/lessons/chrome-devtools-explore-a-site-s-css-with-chrome-devtools-css-overview)
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