I've been wondering this too. Although my wiki isn't having trouble like you describe, I'd still like to make it faster.
I found this
StackOverflow thread which seems to offer some good ideas on where to start. For me, that may mean using
XDebug to profile the PHP performance of my wiki. I haven't tried yet, but as and when I do, I'll write up my experience here. It doesn't look like it's going to serve up ready answers, but it will offer a lot of performance metrics to sift through.