>>The problem is not CPU speed but multiple CPU cores. If you limit ntvdm.exe to run on 1 core only, it will go away.
>
>Is that something new with Skylakes, as he probably is running Has- or Broadwells without a problem, and they are multicores as well ?

No, the problem can show up on all multicore CPUs and related to ntvdm.exe so any dos program affected. If ntvdm.exe runs on one core even w/o affinity, no problem.