My web application starting getting significantly slower recently, and after trying a few things (I use Postgres), I tried a VACUUM (secure in the knowledge that I'd been doing a daily VACUUM ANALYZE and this would have no affect).

Heh.

Actually, the VACUUM ran for about 45 minutes, and performance improved by two orders of magnitude. Yep, I had some web pages that were taking 45 seconds to over four minutes to appear .. I'm now back into the 1-3 second range, much more livable.

I've been away from MySQL too long to remember what the equivalent command (if any) is, but don't forget to prod your database into performing well.