How much of a performance boost can a normal UBB expect to get from a good pruning. I have about 150,000 post right now and it's been a year since I pruned old ones. Will I see any increase in efficiency by deleting old posts?

Performance decreases as more topics are posted in a forum. The number of posts doesn't matter, it's all in the number of topics.

A single large forum (many thousands of topics) can cause a decrease in total performance because of the way file locking works. (A single slow request will block any other requests from going through.)

Unless your board is actually getting slower or you're getting low on disk space, pruning isn't really be required.

Pruning members is actually more effective for a general performance increase... but you only have 800 or so. Like forums, performance really doesn't take a hit until there are many thousands of records in the system.

Thanks Charles, that's good to know info.