Optimize `WP_Theme_JSON::append_to_selector`

[bor0]

On a basic WordPress installation, visiting the frontend page shows about 1104 calls to the function WP_Theme_JSON::append_to_selector.

We find a way to optimize the function by bringing the conditional check one level above. While it increases code verbosity, it also improves performance, as can be seen by the attached images.

[bor0]

Performance measurements of the current function

[bor0]

Performance improvements after the patch - showing nearly a 1% improvement

[bor0]

cc @SergeyBiryukov for visibility.

[SergeyBiryukov]

Nice, thanks!

[spacedmonkey]

This change will need to made in Gutenberg and ported back to core.

See this PR that have already been merged into Gutenberg

​https://github.com/WordPress/gutenberg/pull/47833
​https://github.com/WordPress/gutenberg/pull/50266

These will be backported as part of the WP 6.3 release.

[spacedmonkey]

@bor0 Can you rebase your PR now that [55907] is committed. I think performance issue is fixed, but worth another look. Thanks!

[bor0]

Rebased. Thank you!

[flixos90]

@bor0 Thanks for opening this ticket, I hadn't seen it until now. Note that my PR ​https://github.com/WordPress/wordpress-develop/pull/4380 optimizes the logic around append_to_selector() in a way that no conditions will be needed anymore at all. So if we commit that, we can probably close this ticket here as well.

Also see #58193.

[flixos90]

In 55950:

Editor: Introduce WP_Theme_JSON::prepend_to_selector() to improve code quality and performance.

The WP_Theme_JSON::append_to_selector() method was previously used for both appending and prepending which violated the single responsibility principle. It resulted in additional conditionals which also came at a performance cost, particularly because the method is called over 1,000 times during a regular WordPress request.

With the new WP_Theme_JSON::prepend_to_selector() method, there are now two distinct methods for the two distinct purposes. The now useless third parameter on WP_Theme_JSON::append_to_selector() has been removed (rather than deprecated), which is acceptable given that it is a protected method on a class that is not intended for extensions.

Props bor0, costdev, flixos90, isabel_brison, oandregal, spacedmonkey.
Fixes #58193.
See #58457.

[flixos90]

Indirectly fixed via [55950].