The most common culprit when it comes to slow playback (or laggy playback) is the storage media not being able to keep up with the sustained data transfer required of the source footage. For example, a DV-NTSC clip requires 3.6MB/second for smooth playback. If the storage media cannot maintain that then playback will lag. Of course, you're more likely to have something like 1080p Apple ProRes422 HQ which should=play fine on a standard USB3 external drive (provided that the drive isn't too full), but it's unrealistic to expect it to play from a USB2 drive. Then again, frames sizes are getting to much larger than 1080. The internal Flash stroage of the iMac Pro is exceptionally fast, but more than just playback will lag if the Macintosh HD gets to be too full.