perf(helpers): memoize acf_get_fields() per group

[parisek]

From which project

sloneek, from the profiling that produced #154. Stacked on #154 — the base is perf/memoize-capability-probes, so this diff shows only the new change; GitHub retargets to main when #154 merges.

The finding

getFieldObjectsByScreen() asks ACF for a group's fields once per group per screen. A 90-item menu resolves the same group ninety times.

	calls	time
before	348	26-38 ms
after	21	5-8 ms

That is measured on the work removed. End to end the page moved 16-34 ms across two rounds — the same saving through a noisier instrument, and the machine drifted between them, so the component figure is the one to trust.

Rendered HTML is byte-identical, checked against a control pair of runs of the unchanged code because the page is not deterministic without one.

Why this one needs no invalidation

Field definitions are configuration, not content. They come from the theme's JSON, they do not depend on the page being rendered, and nothing a visitor does changes them. The memo lives for the request, and there is nothing for a hook to notice.

A field value is the opposite, which is exactly why #155 is parked: caching one layer up meant caching formatted output, and formatting runs shortcodes and arbitrary filters that can read the current page. The two look like the same optimisation and are not.

The key carries the blog id and the language for the same reasons the screen memo's does — groups are registered per site, ACFML translates a field's label, instructions and choices. A group carrying neither a key nor an id has no identity to memoize on and is asked every time, rather than sharing an entry with the next anonymous group.

What was tried and did not work

Recorded so nobody repeats it. All measured on the front page, all rejected:

Idea	Result
Prime the post-meta cache for all menu items	Already warm. 0 ms, query count unchanged.
Skip get_field() where the item has no stored meta	206 of 272 calls skipped, time unchanged — ACF already short-circuits.
Prime the term-relationship cache	Already warm, and added 5 queries.
Cache the formatted field payload	#155, parked: formatting is not a function of the stored value.

What remains unclaimed on this path is get_field() for the 66 menu-item fields that hold a value (37-68 ms) and wp_get_post_terms() (9-16 ms, removable only by threading the known menu id through formatFields()).

Tests

Nine, each verified to fail without the memo where it is the mechanism under test: one group asked once, identity by key and by ID, an anonymous group asked every time, separation by language and by blog, a non-array answer normalized, and the flush.