Oh sorry guess I should have given all my specs. Mobo is asus p8z77-v lk, cpu i5 3570k. My ssd has 91.7 GB free. Should I use it as a cache drive as well or would that significantly affect the speed. If so what's the optimum size to use as cache. Btw my ssd is Samsung 84 pro 120 gb.
I would not use your OS drive in an IRST cache setup -- that would adversely impact its performance. The optimal size drive for using to cache would be a 60GB or 64GB drive, as that is how much IRST will support.