Hello, Rocknroll!
I don't know your skills on managing the server, so sorry if I'm writing things that are plain obvious to you
, I am a sysadmin too, so here are some basic tips for slowness, it's not a bad thing to double check them.
Is php caching installed and working well? (On the forum settings' side if it supports it, and also on the server by installing some php caching module, such as APC, or memcached.) Turning the cache on can make a HUGE difference in everything.
The site also has a huge database I guess. I worked with very large databases in the past, and my experience showed that after a reinstall/merge, sometimes the DB server software checks and verifies the whole database for errors, and on large DBs it can take a VERY long time, especially if the server's other resources are also exhausted.
Check if it isn't the database subsystem that takes heavy resources currently.
If you already past these things then consider this post meaningless and accept my apology... I had good intentions with the composition of it.