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WordPress’ Performance Team has published a summary of a core performance analysis they completed in order to identify and prioritize areas for improvement. As part of this process, contributors created a methodology with a standard set of tools that can be used to collect and share profiling data for various components of the application.
The team tested a classic theme (Twenty Twenty-One) and a block theme (Twenty Twenty-Three) configured with the Theme Unit Test data. They tested out of the box functionality, in addition to different scenarios such as a homepage displaying the latest posts, a basic text-only page, a page with a large set of images and default blocks, and a homepage and a basic page with translation.
These tests uncovered numerous performance issues which the team has documented with related trac tickets and detailed in the summary of the findings. The first priority identified for improvement is template loading for classic themes.