February 6th: published using data from WordPress 6.2.0 beta.
March 30th: updated after the WordPress 6.2.0 release.

During the last trimester of 2022, I focused my time looking at server-side performance for WordPress. One of the things I struggled the most was the lack of clear metrics to measure progress, so I decided to track the Core Web Vitals in Gutenberg as a starting point. Initially, server-side metrics like Time To First Byte — aka TTFB. Then, also some client-side metrics. I was inspired by the long history of tracking editor metrics in Gutenberg that Riad pioneered. The performance team was inspired as well, and there is now a dashboard for WordPress server-side metrics.

With this new tool in my toolbox, I wanted to know the evolution of TTFB across the last major WordPress releases. So I prepared an experiment: how long does it take the server to process the homepage for each of the last 4 default themes? I ran the experiment 1 000 times for each scenario and calculated a few metrics related to time per request: median, minimum, maximum, and 75th percentile (see full dataset):

Key takeaways:

• 6.2 is the first major version that improves server-side performance across the board:
  • +25% for block themes in all metrics (median, min, max, 75th percentile).
  • +10% improvement for classic themes (75th percentile).
• 6.1 worsened it for block themes and improved it for classic ones.
• 6.0 and 5.8 took a major hit for every type of theme. 5.7 also had a negative impact.
• 5.9 improved classic themes.

Steps to replicate

In case anyone is interested in replicating the experiment, this is what I’ve done:

1. Cloned the wordpress-develop repository and switched to the corresponding major version to measure.
2. Set WP_DEBUG, WP_DEBUG_LOG, WP_DEBUG_DISPLAY, SCRIPT_DEBUG to false in .env.
3. Started the environment: npm install && npm run build:dev && npm run env:start && npm run env:install.
4. Ran the test: load the homepage. I used this command: seq 1000 | xargs -Iz curl -o /dev/null -H 'Cache-Control: no-cache' -s -w "%{time_starttransfer}\n" http://localhost:8889 | xclip -selection clipboard.
5. Pasted the results in the spreadsheet.
6. Switched to a different theme. Go to step 4.

Coda

WordPress 6.2 brings a great improvement in server-side performance, it’s the first that has had a positive impact across the board. Though great to see, the major improvement has been how having a clear set of metrics has improved the quality of the conversations around performance within the WordPress community.