Are you hitting swap? Generally, the better solution is to avoid that entirely, or at least make it so that things which are swapped out are genuinely not in active use, so that the speed doesn’t matter. Put your money into more RAM.
This is particularly true because while high-end SSD drives may improve performance, cheap ones are very troublesome in this regard.
There is a great article on this week’s Linux Weekly News which I highly recommend reading: http://lwn.net/Articles/428584/. The summary is that cheap drives are very, very sensitive to access patterns, and Linux isn’t currently designed to match that well. Worse, the drives don’t really expose that information in a useful way, so Linux can’t necessarily do the right thing. The best best is to use them with their pre-existing FAT32 filesystems, which are factory-configured to match the drive’s expectations.
Or else you should buy expensive high-performance SSDs — but only when you’re already maxed out on RAM. (And really, at that point, you might strongly consider just getting a newer server which supports more RAM.)