Upon closer inspection in Task Manager, it seems the problem may be related to Windows' memory management, specifically "System and compressed memory" which shows quite a bit of activity when running this game. It's supposed to only kick in when it senses "memory pressure", but the game is barely touching my system ram. 70% of it is free.What's even more strange is that this doesn't cause issues on Nvidia GPUs. Wouldn't be the first time this has come up; Arkham Knight was affected by this under Windows 10 also.
OK so I've now checked it with RTM Windows 10, pre TH2. The same issue exists but without the "System and compressed memory" activity. I now strongly suspect this is a driver issue which would explain why it doesn't happen with my GTX 670 and why it also doesn't occur with AMD or Nvidia under Windows 8.1.
Again, this is across two different systems, same results.