On certain combination of the graphic card and the driver version, the performance of Windows hardware decoding would get slower gradually. The more video Firefox performs in hardware decoding, the worse the hardware decoding performance would be. This issue is tracked by bug 1890622.
This test will reset the same URL on the targeted media element, and measure how long does it take to receive `loadeddata` event that indicates that the first video frame is loaded. Click the button below will repeat 300 times on the above behavior, which would affect the performace of the hardware decoding.
If you want to see the detailed time of decoding every single frame, follow the step2 to step4 to use Firefox Profiler to check the markers.