Since the latest GeForce update, there is a scenario where Nvidia users can find themselves running at 12-20 FPS instead of their usual 80 or 100+ rates. This situation exists with running borderless window mode. For this bug to occur, you also need to be using a frame rate cap of some sort. I think, but am not sure, that reduce FPS while minimized is also a key factor.
For example, I have a 144hz refresh rate monitor as my primary and a 60hz monitor as my secondary. When I run the game borderless, uncapped, no reduction, my graphics card uses it's full frequency and boost clock as expected. For frame of reference, this is 1980MHz on a GTX 3080. My frame rate is often over 170.
If I cap my refresh to rate 72hz in borderless window mode, using the Nvidia control panel, my FPS drops to 12-20fps and my graphics card runs at 900MHz.
If I cap my refresh rate to 60hz in borderless window mode, using the in-game frame cap option, my FPS drops to 12-20fps and my graphics card runs at 900MHz.
If I go fullscreen with the same settings, I get 72fps and 60fps respectively as expected. The graphics card also goes up to 1700-1800MHz speed.
Added wackyness, if I lower the refresh rate of my primary monitor to 72, 90, or 120 in windows and then use the in-game or Nvidia control panel frame cap to the same rate, I do not experience the drop to 12-20fps / 900MHz respectively.
The issue seems to be that the graphics card is not treating the borderless window mode of the game as if it is currently in focus. It isn't prioritizing it and is constantly treating it as if it is minimized. Hence, reduced MHz on the frequency.
Solution:
A.) If you play in borderless window, uncap your FPS. Keep your "reduced fps when minimized" option on because you could burn up your graphics card while the game is minimized.
or
B.) If you wish to play with a cap, make sure "reduce fps while minimized" feature is off. Secondly, reduce the refresh rate of your monitor to what you want that cap to be. Failing to do so will under utilize the graphics card. If you want to play at 90fps, set the monitor to 90 refresh rate. 120fps? 120 refresh rate. You can still utilize Gsync this way.