[2021] "Fastest" speed tied to refresh rate

The fastest game speed does work differently to the others. Essentially it says to the game "run as fast as the hardware allows regardless of the effect on the visuals". You can see more info on that here:
https://forum.paradoxplaza.com/forum/threads/low-fps-on-good-pc.1400522/#post-26681142  

That's odd as my understanding is that Vsync does nothing in borderless or windowed modes, the refresh rate of the desktop is used.