I've noticed slightly longer load times for some games, however, I've not noticed any in-game lag on my PSTV [ENSO 3.65+] using a 1TB 7200RPM drive. Just make sure it has its own power source. Single USB hard drives will not power up without a second source (Y USB cable with the other end plugged into a wall adapter/computer/other USB power source; wall outlet.)
I am having a small issue with incorrect freespace showing up, though. This may be an exFAT issue, though.