Re: bbpress and memcache?
On a single computer system with a fast mysql server and fast disk cache, this typically is not a big problem. As a site has to scale this does become a problem because there is only one mysql source and everything has to be fetched from it.
Interestingly we’re running a WP install with two load balanced app servers and two DB servers… our host has the MySQL tuned up so well that we’re seeing absolutely no advantage when we installed memcached as an object cache. The lesson is: don’t assume, test. Check your queries AND page gen time, then install your proposed enhancements (APC, memcached, whatever) and check your queries and page gen time again… seeing an improvement? Keep the changes, otherwise drop them.