then question

how much posts must have "large database"?
I can't say that it is completely tested, but on my side server load seems to become 10-15% less on 2 forums (one with more then 200000 posts and second with about 2000 posts). May be because on both that forums much more people search then post - not sure...
And yes. Posting takes more time, but it don't "make a index for all posts"

It update existing index.