This was fixed with the first SU10 Beta (1.27.9.0). I was able to pop out as many windows as I wanted with no noticeable performance loss and really awesome performance (45-55 fps, 4K, all ultra). This was both with DX11 and DX12.
Sadly, the follow-up (1.27.11.0) broke this again and it’s now actually worse than it is in SU9. Far more latency and stuttering when popping out windows.
Of course, this seems to be ignored by MS as usual. Adding some votes to the thread below could be helpful to raise attention to this lingering problem.
I’m hoping they can reverse course and revert to whatever they did in the original SU10 Beta, as it was an amazing experience to be able to run the sim at full performance with popouts.