Hello,
I have the same problem too. I spoke to MAXON support and they told me that since the R21, they have changed the way the graphics hardware handles the interface.
They told me that until the R20 it was only the viewport that used the gpu. From R21 onwards, all the interface uses gpu, but it doesn't seem to work well with high hz monitors
i also made a video demonstrating the loss of fps when painting keyframes
Have you found any solution to take advantage of all the hz of the monitor?