Posting this in case it helps someone else.
TL
R - to fix stuttering check all your USB stuff including hubs.
Windows 10. 64 bit installation. Native Instruments interface (ASIO)
I had significant problems with playback. The performance monitor did not show high CPU usage. There might have been a disk usage problem but I freed up space and defragged everything and it didn't help.
I used LatencyMon to monitor performance whist I played a looped part of a mix.
http://www.resplendence.com/latencymon
LatencyMon showed that the USB drivers were laggy.
I unplugged everything USB and and started playback from Reaper. First thing I plugged in was the Native Instruments interface. I found that not all USB ports performed equally.
Having found the port that gave the best playback (i.e. flawless) I then mucked around endlessless with different combinations of port-->hub-->peripheral until I identified the combination that worked.
Lessons:
1. Don't assume it's CPU or disk throughput issues (but check this first).
2. Look systematically at whatever you've got plugged into USB ports.
Posting this in case it helps anyone else.
--Aidan