I agree with Matt here, the only way to get more speed now is a big fat and fast CPU, say six cores or more and definitely as much RAM, and high speed RAM at that, as your motherboard can support.
Might even need a change out of motherboard for the bigger and faster CPU anyway.
As a by the way, although the actual operating system software is held on the SATA mechanical drive, once the system has booted up, the system is actually running from the RAM, it is the taking of the system from the drive and putting it into the RAM that takes the time at boot up.