Dealing with the same for the last few weeks. Works fine on ultra settings with upscaling to 125, until i try to move the mouse. then drops to below 10fps and the gpu utilization in Afterburner shows drops to 0; RDRthread goes to over 200ms. nothing i tried so far helped. if the game is played single-frame so to say, it is fluid. if you want to move the screen and look around - unplayable. i am running ryzen 5950x and a evga 3090 - not overheating and all other games i play are fine on ultra settings.