Description: all Steam and Oculus games show a bad refresh rate on head movement when they are on top. But, if the game is in the background (by pressing the Alt+Tab), the image refresh becomes fluid again. Normally, the problem doesn’t occur in Steam VR Home and Pimax Experience. 

If you tried to adjust "Render quality", turn on "Smart Smoothing", adjust the resolution in Steam VR, and even all the settings in Pitool but with no luck. Please turn off the "Hardware-accelerated GPU scheduling" option and try again. The method to turn on or off Hardware Accelerated GPU Scheduling is shown in the picture below: