For all the good things about the SMF bulletin board, the search facility is terrible. Even the SMF guys say it is pretty bad. The only suggestion they have is to switch to a third party utility that is not real time.
I tried it and it was nasty. Never consistently worked and the SMF folks were no help, nor the third party who wrote the thing.
Basically it is what it is and there really is not much we can do about it.