Hehehe
APC (
http://pecl.php.net/package/APC) is an opcode cache that compiles the forum into an intermediate language. It's a bit dodgy, but when it works, the forum goes about 2.5x as fast and uses less processor time.
We are still hosted at the same place we were at - they suddenly worked out that they had a degraded SAN. Once they fixed that (which just happened to coincide within a week of my submitting a ticket on slow disk performance), our disk speed went up to normal SATA levels. Thus the forum feels snappy. I'm giving them a chance to keep these levels up.
In the meantime, I'm likely to move us to lighttpd with XCache. Lighttpd uses about 1/10th the RAM of Apache for a slightly faster performance level, leaving more RAM for MySQL to cache more of the forum's active working set, thus improving responsiveness again.
thanks,
Andrew