In some respects, you are asking the wrong question. The first question to ask is, what is the resource that is being used most heavily? Is it disk I/O, CPU usage, network bandwidth, or RAM/virtual memory? My guess is disk I/O or RAM.
Once you know what the bottleneck is, then you can figure out how to solve it.
Use iostat, sar, and vmstat to figure out what is going on. I have found sar to be useful, but you have to turn it on first and it is more of a "what happened" rather than "what is currently happening" tool.
