I found that restricting CTU does improve speed a lot and can improve sharpness with lower resolution content... BUT this sharpness improvement was only visible when doing frame by frame comparison AND it came at the cost of more blocking in motion which was significantly more visible in ordinary viewing.
My tests are always done using single pass capped VBR set up for adaptive streaming, so take that with a grain of salt, but I found leaving CTU and merange alone gave me the best results. I found preset slow to be quite tolerable in all cases.
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