I ran Unity off of a hard drive, and there was no stuttering, so storage speed isn't a factor. After all the patches the game does seem to run decently now, though some users have reported performance problems still, though they tend to be people with either weaker CPUs, or slower graphics cards. Even with performance improvements, the game is still quite demanding and does need fairly high end hardware if you want a solid 60FPS on the high settings. Make sure to keep the Nvidia GameWorks features disabled, pretty much all of them will tank your performance with the possible exception of HBAO+.