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I don't wanna start new topic for small question, so I decided ask for help here.
I'm encoding 4k HDR > 1080p HDR with preset Slower, CRF 12-13 with output bitrate 16-17mbps. And my question is how much --merange and --ctu affects to quality?

For example:

--ctu 64 --merange 57 (2.5fps)
--ctu 32 --merange 26 (2.83fps) +13% speed

Avg QP is the same, output file size almost the same (428.2mb vs 428.5mb).

Is it worth spending 13% speed?

Ps: I have i5-9400f 6 core cpu, so just lowering --ctu don't give me boost on speed, all boost from lowering --merange.

My full settings:

--level-idc 51 --sar 1:1 --colorprim 9 --colormatrix 9 --transfer 16 --range limited --master-display "G(13250,34500)B(7500,3000)R(34000,16000)WP(15635,16450)L(10000000,1)" --max-cll "0,0" --hdr --hdr-opt --hrd --chromaloc 2 --repeat-headers --min-luma 0 --max-luma 1023 --no-cutree --no-sao --selective-sao 0 --no-open-gop --no-b-pyramid --no-strong-intra-smoothing --vbv-bufsize 160000 --vbv-maxrate 160000 --min-keyint 23 --keyint 240 --ipratio 1.30 --pbratio 1.20 --deblock -3:-3 --qcomp 0.65 --aq-mode 2 --aq-strength 0.8 --psy-rd 2 --psy-rdoq 2 --rd 4 --limit-refs 3 --bframes 8 --rc-lookahead 60 --subme 5 --me star --ctu 32 --merange 26 --max-merge 4