Theme Showcase: Poor SSR Performance

[Copons]

At the Division Meetup, I had a chance to chat with @noahtallen who noticed some poor performance on the Logged-out Theme Showcase.

Some highlights from the Slack conversation (linked below — please read it thoroughly!):

• fetchThemeFilters and fetchThemeData have a high-performance impact.
• It takes 400ms to fetch the /themes endpoint and 800ms to fetch the /theme-filters one, with the cache helping a lot.
  • Reproducible via https://developer.wordpress.com/docs/api/console/.
• We could create a new endpoint to fetch both in a single call.
• It's possible that the slowness is caused by running format_results in Elasticsearch multiple times for themes that are returned more than once (e.g. because they belong to several filters).

The full context: p1699558844418269-slack-C05U9M2DB0D

[noahtallen]

See also this diff: D128627-code. It doesn't have any solution, but it does annotate some of the code with the problem spots.

[BogdanUngureanu]

Hey @noahtallen. I've also opened a diff that should fix the performance issue for the theme-filters API endpoint.

D130453-code

The performance issue comes from calling the WPCom_Theme_Query for each theme in the list. However, we only need that only for the starter-designs.php API endpoint.

So, to fix this, I've:

• Split the method into two dedicated methods: one for the starter-designs and the one for the ThemeShowcase theme-filters
• Move the logic outside of the WPCom_Themes god class.
• Add more caching (sadly, running the code in the background is not possible because we don't have a persistent cache, that's never cleared)

Based on some tests, on cache miss, for theme-filters, the code runs in about 24ms.

The code should be fine, but I haven't tested it thoroughly since we currently have other priorities (and currently is on hold). Feel free to continue the work, if you have time. Otherwise, I'll continue working probably after the holiday season.