All things can only go as fast as the slowest item in the line. I'm sure you've been behind that guy on the freeway... LOL
With internal to externals drives the slowest link in the chain is often usb2.
With usb3 connections then the 5400 rpm drive would be the slowest link but you should still see good speed; just don't expect anything near 500MB/s the port can theoretically do.