I'm not entirely sure if I know what I'm talking about but would it not be possible to periodically move posts/threads etc. that are older than a certain amount of time (say a year) to a seperate table (or tables) which are then only ever searched when absolutely neccessary?
So regular thread list 'stuff' would only look at the 'current' table(s) wheras the Search might look at both the 'current' and the 'archive' table(s) using whatever SQL magic that requires.
Any time a post is retrieved it would compare its number to the 'last-archived-thread' and based on that decide which table(s) to get it from. For the most part it would only require recent posts so only ever do stuff with the leaner 'current' table(s), thereby making everything faster, finding a cure for AIDS and achieving world peace.
Does that make sense? Am I repeating people? Can I go home yet?!
Hmm. But then posting in a thread from 3 years ago might cause problems, or would it? I don't know.