USB3 operates at higher speeds than SATA can handle, so optimally you should notice no difference between USB3 and an internal drive of the same type. In my experience with my own drive, I've noticed this to be true. So basically if you have a external 7200RPM HDD it should be as fast as the HDD company has specified.
SATA max. Mb/s= 550
3.0 max. Mb/s= 625
3.1 max. Mb/s= 1250 MB/s
3.2 max. Mb/s= 2500 MB/s