It's been already exposed many times, that slower modes of CRF using x265, are desperately trying to extract quality from the video stream repeating the same algorithms again and again, but most of the times they increase the size of the encoded video without actually increasing quality.
It would be wise to stay at "x265 slow" and not going slower, but nowadays it would be even wiser to try a hardware encoder of H.265 like Turing's which produces low size, good quality (even better in many cases than x265) and extremely fast H.265 encodings.
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