After nearly 13 years, Microsoft seems to have addressed an oft-ignored issue when it comes to how Windows deals with multi-monitor refresh rates. Till now, Windows 10's Desktop Window Manager (DWM) caused stutters and frame-skips when running multiple monitors with mismatched refresh rates. Windows 10 2004 20H1's DWM, which supports hardware-accelerated GPU scheduling in WDDM 2.7, finally seems to have corrected this issue as demonstrated by a user on Reddit.
https://www.notebookcheck.net/Windows-10-2004-20H1-finally-gets-multi-monitor-refresh-rates-right.454143.0.html
There's a big BUG on this feature.
If you have a monitor 144hz and a second display, even If the second display is off, some games will force the fps for the lowest refresh rate of the monitor (for example , mine is 144hz, the lowest rate is 60hz on this monitor), when I open some games, like Crysis 2, Crysis 3, something makes the fps forces to 62fps, and it doesn't matter If Vsyinc is on or off., It's a bug on refresh rate that attacks some games.
So I do NOT recommend to update to windows 2004 build.