[wp-trac] [WordPress Trac] #63858: Trigger the wp_cron action from the shutdown hook instead of the init hook to reduce TTFB (⏱️ Time To First Byte) and improve performance 📈

[WordPress Trac]

#63858: Trigger the wp_cron action from the shutdown hook instead of the init hook
to reduce TTFB (⏱️ Time To First Byte) and improve performance 📈
-------------------------------------------------+-------------------------
 Reporter:  pmbaldha                             |       Owner:
                                                 |  westonruter
     Type:  enhancement                          |      Status:  accepted
 Priority:  normal                               |   Milestone:  6.9
Component:  Cron API                             |     Version:  2.1
 Severity:  normal                               |  Resolution:
 Keywords:  has-patch needs-testing needs-dev-   |     Focuses:
  note has-unit-tests                            |  performance
-------------------------------------------------+-------------------------
Changes (by westonruter):

 * keywords:  has-patch needs-testing needs-dev-note => has-patch needs-
     testing needs-dev-note has-unit-tests
 * owner:  (none) => westonruter
 * focuses:   => performance
 * status:  new => accepted


Comment:

 I'm testing [this new PR https://github.com/WordPress/wordpress-
 develop/pull/10205] with the following plugin active:

 {{{#!php
 <?php
 /**
  * Plugin Name: Cron TTFB Fix Test
  */

 add_action(
         'http_api_debug',
         static function ( $response, $context, $class, array $parsed_args,
 $url ) {
                 if ( str_contains( $url, 'wp-cron.php' ) ) {
                         error_log( 'Cron request spawned: ' . $url );
                         sleep( 1 );
                 }
         },
         10,
         5
 );

 add_action(
         'init',
         function () {
                 if ( defined( 'DOING_CRON' ) ) {
                         error_log( 'Doing cron!! Spawned URL: ' .
 $_SERVER['REQUEST_URI'] );
                 }
         }
 );
 }}}

 Note that it adds 1 second of additional latency on purpose to the
 `wp_remote_post()` call to simulate the HTTP API not being correctly
 handling the `timeout` and `blocking` params.

 To get a baseline TTFB for the homepage:

 {{{
 $ curl -o /dev/null -s -w 'TTFB: %{time_starttransfer}s\n'
 http://localhost:8000/
 TTFB: 0.174550s
 }}}

 So a homepage request is 174 milliseconds.

 Then I test spawning cron. On `trunk` I run the following from the command
 line:

 {{{
 npm run env:cli cron event schedule foo '+1 second';
 sleep 2;
 curl -o /dev/null -s -w 'TTFB: %{time_starttransfer}s\nTTLB:
 %{time_total}s\n' http://localhost:8000/
 }}}

 This results in:

 {{{
 Success: Scheduled event with hook 'foo' for 2025-10-10 00:13:59 GMT.
 TTFB: 1.210728s
 TTLB: 1.211613s
 }}}

 Note the TTFB is now 1 second worse because that HTTP request took longer,
 and the Time To Last Byte (TTLB) is almost identical to the TTFB.

 The expected "Doing cron!!" message also appears in my error log.

 Then I switch to the branch in the PR and re-run the above command:

 {{{
 Success: Scheduled event with hook 'foo' for 2025-10-10 00:14:43 GMT.
 TTFB: 0.185948s
 TTLB: 1.196698s
 }}}

 I again see “Doing cron!!” appear in the error log.

 Note the TTFB is back down to the original range at 162 ms, even though
 the TTLB is unchanged.

 All this indicates that cron is successfully moved to shutdown and that
 this improves TTFB.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/63858#comment:11>
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