After 2~3 seconds of playback, Fusion purge its cached memory and start re-rendering. This is strange behavior. I figured out that the combination of Sapphire 14.51 and Fu v17.2.2 was the cause. v16 is fine. V17.2.1 was probably not a problem either.
I just put S_LensFlare next to BG. After 2~3 seconds of playback, Suddenly the frame rate drops. This is because the cache is being cleared and re-rendered. It also shows up in the memory graph in Task Manager.
Please check it out.