I'm somewhat skeptical that a CRF encode would get that much slower based on content type. A soft film should have a lower average bitrate, and thus less CABAC. And trigger early exits more often. I'd guess the reported speed differential is more likely from a slower decode or preprocessing (like if you're only doing degrain on softer sources).
Differences between medium and slow that might cause bigger perf differences than quality differences
* --lookahead-slices 4 instead of 8 could slow things down if you have lots of available cores.
* --rect isn't likely to change quality that much, just increase bitrate some
The other differences seem more likely to impact quality AND bitrate.
Personally, I feel --preset slower is where HEVC starts to really strut its stuff.
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Ben Waggoner
Principal Video Specialist, Amazon Prime Video
My Compression Book
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