[Zope] Help: Zope is extremely slow

Our user base has grown significantly in the last few weeks. Unfortunately at hours of peak usage Zope crawls to a halt and won't even serve up pages. we have NUMBER_OF_THREADS=4 would increasing this help? Actually when we I check the processes (with ps -fe | grep py) I only see two the root thread and the thread that runs as nobody? Anybody have any suggests to help the performance of the site? Dan