`ACF_Local_JSON::scan_files()` re-reads and re-decodes the whole `acf-json` directory four times per request

[jpmckinney]

Describe the bug

ACF_Local_JSON::scan_files() performs a full scandir() plus file_get_contents() and json_decode() of every file in every load path, and caches nothing between calls. It is called four times on every request, once per internal post type:

Caller	Post type
includes/local-json.php:310	acf-field-group
includes/local-json.php:331	acf-post-type
includes/local-json.php:352	acf-taxonomy
pro/post-types/acf-ui-options-page.php:307	acf-ui-options-page

Each call reads and decodes the entire directory before get_files() filters the result down to the requested post type. On a site that stores only field groups, 3 of the 4 passes read every file and match nothing.

This runs on acf/include_fields, acf/include_post_types and acf/include_taxonomies, so it applies to front-end requests as well as admin.

To Reproduce

Make a request.

On my site with 69 field-group JSON files, 0.38 MB total, on PHP 8.1 with OPcache warm:

files: 69, 0.38 MB per pass
4 passes (current behaviour): 18.9 – 33.8 ms
1 pass  (sufficient):          3.3 –  7.1 ms
wasted per request:           15.6 – 26.7 ms

That's 276 file_get_contents() + json_decode() calls per request, of which 207 are discarded.

My sampled PHP stack traces (PHP-FPM slowlog, requests over 1 s) put local-json.php:409 and local-json.php:312 among the most frequently caught frames, which is what prompted this investigation.

Expected behavior

Memoise the directory scan.

scan_files() already assigns $this->files before returning, so the results are being kept; they are simply rebuilt on every call:

// includes/local-json.php:381
public function scan_files( $post_type = 'acf-field-group' ) {
    if ( is_array( $this->files ) ) {
        return $this->get_files( $post_type );
    }
    // ... existing scan ...
}

Invalidation within a request should not be needed: update_field_group() and friends write a file and can reset $this->files explicitly if required.

An alternative that avoids reading unrelated files at all: JSON files are named {key}.json, and each internal post type has a distinct $post_key_prefix (group_, post_type_, taxonomy_, ui_options_page_), so glob() on the prefix would identify candidates without decoding anything.

Version Information:

• WordPress Version 7.1
• PHP Version 8.1
• ACF Version PRO 6.8.8
• Browser Safari 26
• MySQL 8.0

[anthonyburchell]

Hello, I verified your report and we have a few directions forward. I'll discuss with the team about getting it in an upcoming release. Thank you for the report!