Does seem to be a recurring problem, understandable if it happens when the site is busy but it can run slow anytime, just seems to be a problem that's developed over the last 6 months :?
Don't usually complain about these kind of things seeing as I do IT myself but this problem is getting bloody frustrating. The whole site vanished for around 10 mins (about 10.30 - 10.45pm) and has been dog slow on and off all day.
Would it be an option to start pruning users and forum/archiving a bit to reduce the load of things that are less used?
I'm not super-techy, but I think the normal accepted load time for webpages is less than 5 seconds for people who are not familiar with a webpage. I've had 5 minute waits today also trying to submit a post - I know that this is a super forum, so I have patience, but it can sometimes be wearing a bit thin when it gets that bad.
Technically pruning users would have no effect as these reside on the database which have no impact on the slow speed if the forum as if I am not logged on, it's still slow. Although its good housekeeping to do this.
I would say you need to check server logs, and clear the cache.
What version of phpBB is this? Is there a new version, maybe schedule an upgrade to the forum software. Who are you hosted with? Check with them that you are not being capped on bandwidth? This would slow it down I you have reached your limit and maybe need to up your hosting package?