Comment by microtonal

This. The post immediately reminded me of Win4Lin 9x (the version before it became just another boring VM) and SCO Merge. It was insanely fast, even on the hardware of the day.

The Wikipedia page is not verify informative and presents it as a regular VM (possibly mixing up 9x and later versions that run the NT line of kernels). The manual is a bit more informative about the tech:

ilab.usc.edu/packages/special/Win4Lin-3.0.1-manual.pdf

I’m a bit surprised it hasn’t been mentioned a lot in the comments. Maybe it’s a bit too old for most people here (Linux in the late 90ies/early 00s was a much smaller community)?