Are you by any chance running AVG or Avast antivirus on the Windows 10 host?
There has been a spate of slow VMs few months ago and a lot of commonality was AVG or Avast.
Avast and AVG seem to have some hardware virtualization scan that seems to be causing the problem.
If you are using either have a look at this thread, there are steps to disable the hardware virtualization feature of Avast and AVG.
XP VM suddenly slow, Win 7 fine
