You don't necessarily need to port apps to run under Windows 10 on ARM. It runs native Windows apps in a 32-bit Windows emulator, which is it's big restriction - it doesn't support 64-bit Windows apps:
https://docs.microsoft.com/en-us/windows/uwp/porting/apps-on-arm
As for Apple/ARM native...
To be completely accurate, it would be emulated AND virtualized in that scenario. It's also possible that Parallels could emulate the x86 hardware layer and allow x86 Windows installs, but yeah, that performance thing is huge.
I'm wondering if, in the short-term, we'll see Apple use ARM CPUs in...
I can tell you this, Ron - Parallels is doomed if they can't find a way to virtualize Windows on ARM-based Macs. It's the #1 reason people buy the app in the first place, particularly in the business world. I work at the largest public university in the U.S. and have installed Parallels on macOS...