[wp-trac] [WordPress Trac] #54423: Add Site Health test for page caching (advanced cache)

[WordPress Trac]

#54423: Add Site Health test for page caching (advanced cache)
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 Reporter:  westonruter  |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  Site Health  |    Version:
 Severity:  normal       |   Keywords:
  Focuses:  performance  |
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 A [https://make.wordpress.org/hosting/handbook/performance/#full-page-
 cache best practice] for performance is to enable full page caching for a
 WordPress site. Doing so reduces the time to first byte (TTFB) and thus
 directly impacts Core Web Vitals. This is especially important on shared
 hosts which have limited compute power, while also protecting a site from
 going down due to traffic spikes.

 As with XML Sitemaps, it may make sense for WordPress core to include a
 basic full page caching solution out of the box which can be extended by
 the many plugins that provide page caching today. In the meantime, there
 should at least be a Site Health test with a low-severity `recommended`
 status to point users to where to go to enable page caching. Since hosts
 often have built-in caching solutions in place, the Site Health test
 should be easily filterable so they can provide a link to their specific
 dashboard or documentation to enable caching which may not be a plugin at
 all but rather a reverse proxy like Varnish.

 In the context of the AMP plugin for WordPress, we've already developed
 such a test which could be adapted for core. See
 [https://github.com/ampproject/amp-wp/pull/6456 amp-wp#6456]. It's an
 async test which issues three loopback requests to the homepage, and the
 test will pass if responses include either of the following headers:

 1. `Cache-Control` with non-zero `max-age`.
 2. `Expires` with a future time.
 3. `Age` with a positive number (as is set via a proxy like Varnish).

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Ticket URL: <https://core.trac.wordpress.org/ticket/54423>
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