Performance issues with `wp_common_block_scripts_and_styles`

[spacedmonkey]

In function wp_common_block_scripts_and_styles is bad for performance. This is because it does a lot of file operations. Uses glob and file_get_contents. This does this for styles are may not even be loaded on a page.

[spacedmonkey]

There is a PR with a fix here. ​https://github.com/spacedmonkey/wordpress-develop/pull/4

This ticket is blocked by #58528

Trac ticket: https://core.trac.wordpress.org/ticket/58560

[spacedmonkey]

Benchmarks, 1000 runs on TT2.

	Trunk	PR
Response Time (median)	126.62	114.16
wp-load-alloptions-query (median)	1.42	1.41
wp-before-template (median)	47.75	48.31
wp-before-template-db-queries (median)	6.45	6.43
wp-template (median)	73.97	61.42
wp-total (median)	121.88	109.85
wp-template-db-queries (median)	3.95	3.91

The theme has to have add_theme_support( 'wp-block-styles' ); to see the benefit, so using TT2 for the benchmarks is important.

[flixos90]

Thanks @spacedmonkey for looking into this. I also ran a set of benchmarks and can confirm a notable performance impact, which makes this worth considering even this late in the cycle. The numbers I see are even more impactful, resulting in a crazy 35% performance improvement (wp-total metric) when using TT2. I can also confirm that this performance improvement only applies to block themes that have theme support for "wp-block-styles".

​See this spreadsheet for the full benchmarking results.

With that said, I am personally not familiar and comfortable enough with the underlying logic to approve the PR by myself. It would be great to have someone else more familiar with the underlying logic review. Maybe @noisysocks @oandregal?

I've done some search, but it's difficult to pinpoint who was involved in this work, given all the moving code around between core and gutenberg. Playing by ear and pinging @aristath, as he has worked in this area, in case he can provide feedback.

[spacedmonkey]

Testing instructions.

1. Activate TT2 theme.
2. Create a post.
3. Insert the following blocks audio, code, gallery, image, quote, search.
4. Add title and publish.
5. Navigate to post on FE.
6. View source of page.
7. Search for wp-block-<block_name>-theme-inline-css.
8. Ensure that all of the block listed above have inline theme styles.

@oandregal Testing details

Thanks for the PR @spacedmonkey! I left one nitpick that can be addressed here or in a follow-up "catch-all" commit, so it's not a blocker.

@costdev Feedback complete.

[spacedmonkey]

In 56064:

Script Loader: Fix performance issues in wp_common_block_scripts_and_styles.

In [52069] the function wp_common_block_scripts_and_styles was changed load individual theme stylesheets, if the current theme supports block styles and loading separate core block assets. To do this, the function calls many expensive file operation functions, such as glob, file_exists and file_get_contents. This is wasteful, as these functions are loaded on every page request, even request that do not include blocks, like REST API calls. In [56044] all core block styles are registered in a single place. In register_core_block_style_handles calls glob to get all css styles in block directories. While registering style and editor styles, also register block theme styles, under a new style handle. Example wp-block-avatar-theme. If the current theme supports block styles, also request the block to enqueue the theme style on the front end. As these new stylesheets have a path attribute set, the function wp_maybe_inline_styles will automatically inline the styles for you.

Props spacedmonkey, flixos90, oandregal, costdev, audrasjb, mukesh27.
Fixes #58560.

Committed ​https://github.com/WordPress/wordpress-develop/commit/30d6172eed95667020e7de6c29f6997e1e50e2a3