Stable working build, Gaming performance is mud

I didn't really build this machine for Gaming but would like to troubleshoot some to get some better FPS.

i5 4670

8GB DDR3 1600

MSI H81 Motherboard

Zotac 750TI 2GB

The machine is very snappy and stable but gaming performance is very sluggish. Most games I have to run on low/med just to be playable. On windows the same games on this 750ti would run without a hitch. What might I be missing?

The games I'm playing are older and not very demanding BTW (minecraft, trove, diablo 3, WoW, CSGO)