Performance improvements for 6.9

[dmsnell]

This is a general tracking ticket for various performance improvements in the WordPress 6.9 release cycle.

Trac ticket: Core-63755
Replaces #9307

This patch introduces a speedup for the seems_utf8() function. Currently, the function scans every byte in user-land PHP code. This is prohibitively slow. In this change, spans of ASCII bytes are fast-tracked through the use of strspn(), which can scan valid UTF-8 single-byte characters at around 3600 MB/s instead of around 300 MB/s as is the case with the existing code.

[sachinrajcp123]

Add targeted caching in get_post_status() and get_userdata() to reduce repeated object lookups and improve overall performance.

Closing in favor of #9317 where seems_utf8() is deprecated.

Trac ticket: https://core.trac.wordpress.org/ticket/63755

This PR makes a minor update to the way default values are assigned for custom template properties in the get_custom_templates method, using the null coalescing operator for improved readability and consistency.

Is Core-63755 the right ticket? This doesn't seem to be a performance improvement.

Closing in favour of #4886

[westonruter]

It doesn't seem like there is anything specific being tracked by this ticket anymore. I suggest closing in favor of tickets for specific improvements.

[dmsnell]

I don’t foresee any new performance-related tickets, so I’m okay closing it.