bbPress.org

Apache, both 1 and 2 are based on old slow code. You can spend hours fiddling with performance settings and barely fix the problem. They don’t do well for very active sites and apache can be easily ddos’ed. You can either throw more hardware at the problem or replace apache when the loads get too high.

LiteSpeed is literally twice as fast/half the load as apache (it’s also a bit expensive for the commercial version but there is a 5 account version available for free). Additionally it can plow through many kinds of ddos attacks. It’s a piece of cake to install and configure (GUI based).

The free Nginx is also twice as fast as Apache but LiteSpeed can use Apache’s httpd.conf and .htaccess files so it’s very easy to install (10 minutes tops) and you can even switch back to Apache if so desired. Nginx requires a complete rewrite of any mod_rewrite rules, etc. and requires technical skill to install/operate.

Matt is slowly converting some of Automattic’s server from LiteSpeed to Nginx to keep the ideals of open source but LiteSpeed is still too good and has no serious competition as far as compatibility so I bet he will probably continue to run it on some servers for years.

As far as opcode caches I am surprised you had trouble with eaccelerator which tends to be the most compatible/easiest to install. APC does have some quirks but it’s being adopted by PHP and may become “built in” in some future version.