A forum breaks the rule every other WordPress site follows. The busiest pages change constantly, most visitors are signed in, and the thing people do most is post.
Page caching barely helps here. What matters is how fast a write lands and how fast a thread renders afterwards.
Every reply is an insert plus a handful of updates to thread and forum counters. The database is tuned for that mix rather than for a read-mostly brochure site, so a busy thread does not slow down the whole board.
Forum search is the single most expensive query a community runs, and it runs constantly. Indexing and query limits are set so one person searching does not stall everybody else posting.
Members are logged in, so page caching does not apply to the pages they actually use. Those come from the object cache and a database that was expecting them.
A spam wave is a write storm plus a moderator trying to work through it. Admin capacity is kept out of the front-end pool so the queue is workable exactly when it is full.
Bulk moderation is one click away from deleting the wrong thing. Daily backups mean a mis-clicked mass delete is recoverable rather than final.
Forums do not fail by getting slow for anonymous visitors. They fail when posting is sluggish, search times out, or the moderation queue becomes unusable during the exact hour it fills up.
The number that matters on a forum is how long a reply takes to land while a few hundred other people are reading and writing at the same time.
A forum is the one WordPress workload where the write path matters more than the read path, and every row here follows from that.
| Setting | What we do | Why |
|---|---|---|
| MySQL configuration | Tuned for a write-heavy mix | Every reply inserts a post and updates several counters, which is the opposite of the read-mostly profile most WordPress tuning assumes. |
| Search queries | Indexed, with a bounded result set | Forum search runs constantly and unbounded, and it is the query most likely to take the whole board down with it. |
| Page cache | Anonymous thread views only | Members are signed in, so the pages that carry the community are pages a cache is never allowed to serve. |
| Admin capacity | Held outside the front-end pool | A spam wave makes the moderation queue busiest at the exact moment the front end is under load. |
| Revision storage | Capped on posts | Edited replies accumulate revisions faster than articles do, and the table grows until queries against it slow down. |
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