short version / tl;dr:
basically the situation with multi-monitors with different refresh rate was improved quite some time ago, but some issues remained... and there are various factors to it.
also, a certain windows update is supposed to fix or improve things further. i don't think that has made it to public/mainstream release for everyone yet. It might be available with Windows 11 24h2
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i've talked about this in more detail somewhere else. i'll copy/paste
the long version below:
windows 10 2004 / 20H1 made things better for people with multi-monitors...
https://www.reddit.com/r/Windows10/comm ... resh_rate/
the situation with displays using different refresh rates didn't exactly get fixed for good or for all scenarios though... and for quite some time now... the dwm (desktop window manager) has been composing at the rate of the main display in windows.
the mouse cursor is special though as it's not normally composited by the dwm but by a dedicated hardware cursor plane (which is also basically why we could see content on a side display dropping frames while the mouse cursor appears unaffected on that display)
and i saw that still being the case in my testing with the latest public version of windows 11 23H2 i got available (using windows 11 education 23H2 OS build 22631.3447). also, you can use intel's console presentmon to test that as presentmon can show the fps of various running processes etc
the issue then normally manifests on content that you're running on a side (non-main) display. if you're playing a game on a side monitor, you may notice that vsync is operating based on the max refresh rate of your main monitor (instead of being based on the max refresh of the side monitor) at least if dwm composition is present with the game. the solution (for the vsync situation) can be to simply run the game on the main display... or playing a game (or running an app) either with legacy flip or with independent flip; those presentations can allow you to run a game on a side monitor without the game using the main monitor's max refresh rate for the vsync cap if you're using vsync.
in fact, i've even been able run 2 games at the same time with both using independent flip presentation and both even using VRR independently; each with its own fps and reported refresh rate/vblank counter; one game was on the main display while the other was on the side display. i got that working fine. i wasn't quite able to get 2 FSE (legacy flip) games running like that though because the FSE game kept minimizing...
generally i'd still recommend keeping the game in the main display though. games often by default launch on the display that's set as the main display in windows and it may be faster that way, plus moving a game's window to another display (that can also be done using winkey + shift + arrows keys with some games and other methods) may sometimes cause the game to get stuck on composed flip presentation
and if you use an nvidia gpu and like using nvidia reflex... keep in mind that one of the things nvidia's reflex low latency mode does is lower a game's fps below the display's max refresh rate if you're using gsync + vsync on (and reflex low latency mode does that to keep you within the vrr range, lower latency, and account for some frametime variance basically), BUT reflex low latency mode does that based on the max refresh rate of the display you have set as the main display in windows (even if you're using legacy flip or independent flip), so you may notice that won't work correctly or at all if you're not running the game on your display that's set as the main display in windows.
beyond the scenarios i already mentioning involving a game on a side display, stuff like youtube videos (at least when they're presenting with dwm composition) on a side display may also get rendered with lower fps based on the content that's running on the main display... and that isn't just with a windowed/borderless or FSO'd game running; it happens with FSE too (the youtube video on the side display can be affected by a lower fps from a game that's running in FSE with vrr on the main display).
that said, since last year... microsoft has further improved refresh rate logic to allow the OS to manage different refresh rates on different monitors depending on the refresh rate for each monitor and content shown on the screen (which previously at least wasn't possible if the apps were being composited by the dwm). microsoft noted that this will help most with refresh rate-dependent multitasking; like playing a game and watching a video at the same time.
i don't think that has made it into the current latest public/mainstream release of windows 11 for everyone yet... since very recently i was still able to reproduce the situation where my youtube video on a side display in certain cases would drop frames while having a game on my main display. anyway, it's something to look forward to i guess. this was mentioned in the windows insider blog:
We have improved refresh rate logic to allow different refresh rates on different monitors, depending on the refresh rate for each monitor and content shown on the screen. This will help most with refresh rate-dependent multitasking, like playing a game and watching a video at the same time.
https://blogs.windows.com/windows-insid ... ild-25915/