Improve performance of WP_Theme_JSON::get_merged_data method

[oandregal]

See:

• Performance issue Performance regression in WP 6.1 for theme.json processing gutenberg#44772
• Trac ticket https://core.trac.wordpress.org/ticket/56467

What

Follow up to #3418

Why

We want to squeeze as many ms as possible.

How

In the WordPress 6.1 cycle, WP_Theme_JSON_Resolver::get_merged_data method has become a hot path that is called many times. By improving small things that are repeated multiple times, we get more performance wins. This PR changes this code:

public static function get_merged_data( $origin = 'custom' ) {
	// ...

	$result = new WP_Theme_JSON();
	$result->merge( static::get_core_data() );
	$result->merge( static::get_block_data() );
	$result->merge( static::get_theme_data() );

	// ...
}

to:

public static function get_merged_data( $origin = 'custom' ) {
	// ...

	$result = static::get_core_data();
	$result->merge( static::get_block_data() );
	$result->merge( static::get_theme_data() );

	// ...
}

As a result, this reduces the number of calls of the low-level WP_Theme_JSON->merge method, with the corresponding performance improvements.

Performance improvements

Method	WordPress 6.0	WordPress 6.1 RC1	PR 3441 (based off trunk@54506)
total time	746ms	877ms	794ms
WP_Theme_JSON_Resolver::get_merged_data	9ms (27 calls)	146ms (98 calls)	55ms (98 calls)
WP_Theme_JSON->merge	25ms (130 calls)	114ms (512 calls)	74ms (414 calls)

See #3418 for instruction on how to reproduce these numbers yourself. Alternatively, you can import the following cachegrind files to visualize in the tool of choice (kcachegrind, qcachegrind, webgrind, etc.):

• WordPress 6.0: cachegrind.out.6-0.gz
• WordPress 6.1, RC1: cachegrind.out.6-1-rc-1.gz
• This PR, based off trunk@54506: cachegrind.out.6-1-pr-3441.gz

Test

• Make sure automated test pass.
• Manually test that global styles still works as expected by using the TT2 or TT3 themes and updating some block styles via the global styles sidebar.
• Use a localized WordPress and verify that translations for colors are still applied.

[oandregal]

I've rebased this PR to be based on trunk@54506 and recalculated data for WordPress 6.0, WordPress 6.1 RC1 and this PR.

[oandregal]

cc @ockham @hellofromtonya this is ready for review/merge.

---

There are some failures in the PHPUnit Tests / 8.2 on ubuntu-latest job. These are totally unrelated to this PR and I don't know why they fail. They say things such as:

For automated test runs, this test is only run on trunk
This test requires the index to be truncated.
Rendering PDFs is not supported on this system.
The image editor engine WP_Image_Editor_Imagick is not supported on this system.

I'd appreciate if someone could re-run these or provide help on what's happening to unblock this PR.

(Edit: https://core.trac.wordpress.org/ticket/56817 investigates this issue)

[mukeshpanchal27]

Thanks @oandregal This change improve the performance in my test.

[aristath]

Nice! LGTM 👍

[oandregal]

Rebased from trunk to address the environment failures. I've seen some test have been disabled since this was reported.

[jorgefilipecosta]

Committed to trunk in cb98544. We should leave the PR open until there is a 6.1 branch and we confirm the commit is there (because the branch was created after the commit) or the commit is not there, but we do the backport.

[desrosj]

No need to leave this open. The 6.1 branch will be copied from the latest state of trunk, and the changeset will be included.

[mcsf]

Tested, works well. A worthwhile improvement for the RC. 👍

[oandregal]

I'm proposing we revert this change at #4145 as it introduced a bug.