I wonder. Have you changed anything since installing new version of SMF? After upgrade everything was initially OK. So, either something happened later, or it took some time for the problems to grow up to the point when they started to be hard to miss. Whenever I see slow responses I attribute them initially to the web having hiccup in general - it happens now and then, so could be I have missed early symptoms of CF/SMF having problems.
If that's SMF - perhaps switching to other forum software will help? I know - that means troubles and it can be simply impossible if conversion of the database is not supported, but perhaps it is worth trying?