You Should Use a Persistent Object Cache: What WordPress Site Health Actually Measures

Site Health recommends a persistent object cache when your site crosses a size line, not when it is slow. Any one of seven counters is enough: a multisite network, more than 500 autoloaded options or 100,000 bytes of them, or 1,000 rows in the posts, comments, options, terms or users table. Revisions and media count as posts, so small sites trip it often. An object cache also needs a cache server, and a plugin alone cannot provide one. Decide first whether your traffic is the kind an object cache speeds up.