You'll likely experience slowdowns, but since the framerate is already so bad it shouldn't matter too much. Probably better to just run it off of your hard drive though.
USB 3.0 can handle up to 5gbps (640MBps)—over ten times as fast as the 2.0 maximum. most sata ssd are only in the 400 to 550 MBps, you should be just fine. in fact it would make taking star citizen with you a breeze. I've heard of people doing this with steam libraries.