I had the problem from before and I recently upgraded to 5090 etc, and somehow I have the same issue.
It would start with 210 fps and after 1 or 2 hours, fps will drop to around 100. This time, I have hardware monitoring and can see that my GPU seems to ‘get lazy’ after some time (low temp and power consumption).
I am not using the strider pet but I am USING other MTX pets; did not lock fps either. Plz help… other games run smoothly for me.