First use a tool like ab to see what your site can currently handle. From that monitor the requests to see what's taking up most of the time (NewRelic is one t

First use a tool like ab to see what your site can currently handle. From that monitor the requests to see what's taking up most of the time (NewRelic is one tool that can help with that, or just analyzing your log files ia fine too). For me, I've noticed caching database queries or just caching entire pages helps the most. Then rerun the ab tool to see how much you've improved. Repeat as necessary.