I was having a problem that my CPU was on 100% and the GPU was like 70%. Due to that I was having shutters and a bad performance with fenix. To solve this problem I installed “MSI_util_v3” and set on the RX6600 options (my graphic) the MSI square to on and the interrupt priority to “HIGH”. I don’t recomend you to use more options of that app. If you had good performance in other games and now you are having bad performance just set the interrupt priority to “UNDEFINED” 
It solved all of my problems and I hope it can help yours too!