Improve performance of get_block_theme_folders

[spacedmonkey]

The function get_block_theme_folders can be called a lot in a template load. This function contains 2 file_exists checks. This can be slow, as it is accessing the file system.

[swissspidy]

is_dir() is much faster than file_exists for checking directory existence. On the other hand, file_exists() also checks if the directory can be accessed. See ​https://bugs.php.net/bug.php?id=78285

So replacing file_exists( $theme_dir . '/block-templates' ) with is_dir( $theme_dir . '/block-templates' ) would be faster. Maybe with is_dir( $theme_dir . '/block-templates' ) && is_readable( $theme_dir . '/block-templates' ) to have the same effect.

---

Aside: makes me wonder why the following code uses all three checks:

​https://github.com/WordPress/wordpress-develop/blob/c40e0c6f816ed27ba8b75462632498a36f624181/src/wp-includes/block-patterns.php#L374-L377

That seems like overkill.

Trac ticket: https://core.trac.wordpress.org/ticket/58319#ticket

[spacedmonkey]

@swissspidy I have looked into is_dir. It is faster, but not by much. It this still the same code that is 20+ per page and it results in overhead.

[spacedmonkey]

For extra context [55236] and #57114.

[spacedmonkey]

Benchmarks run 1000 times against TT3 theme.

	Trunk	PR
Response Time (median)	127.75	118.17
wp-load-alloptions-query (median)	5.06	5.09
wp-before-template (median)	64.99	61.77
wp-before-template-db-queries (median)	5.67	5.28
wp-template (median)	57.21	51.32
wp-total (median)	123.3	113.31
wp-template-db-queries (median)	3.38	3.06