Seems to me the quality and res of the original file is what makes the time vary that is why clipping the black edges speeds it up less blocks to analyze but making the new video a very high res seems to slow it down almost 2x sometimes maybe because of vram restrictions? idk
Example- all from the same 720p file w/2080ti:
1080p 0.60fps
200%/1440p 0.60fps
4K 1.16fps
8k 1.26fps
Example- all from the same 480p file w/2080ti:
200%/~720p 0.13
1080p 0.23
4k 0.26
8k 0.41
200% & clip black edges = quick
