Don't get confused between --ctu and --min-cu-size
https://x265.readthedocs.io/en/defau...on-min-cu-size
You probably don't want to mess with this. Use the presets. 8 is the default for everything except ultrafast.
It sets the minimum possible coding unit size. The HEVC standard lets you go as small as 8x8 and the encoder will do this when it makes sense.
If you force this to be higher then you can go faster in some cases because you don't even evaluate 8x8. In practice, you will usually want to have some 8x8 CUs though, so forcing the encoder to never consider this is a bad idea.
--ctu on the other hand is a super important setting. The default (64) makes sense most of the time. Lower values can preserve a bit more fine detail in still images and can be more parallel (since you're processing a larger number of smaller CUs), but the 64x64 CU is a HUGE deal in HEVC and helps a ton overal for most content, especially 4K, and especially when you compare clips in motion vs still images. The reduction in blocking is great!
I suggest always leaving this at 64 unless you need to go fast, in which case you should be using the presets anyway
