I have installed APC, but I am not observing as big an impact on performance as I expected. Actually, I'm not sure I'm observing any benefit, though maybe it's just because the cpu time spent in interpreting php code is negligible compared to the execution of my own code.
So I'm wondering: is it perhaps because I'm running PHP as an apache module (as opposed to fastcgi) and/or because php is using prefork (as opposed to worker)? Does any of these somehow defeat the performance improvement that could be achieved by means of APC?