x264: Why is it so much slower on higher bitrates?

[pepelugil]

Hi,

maybe this is a silly question previously answered, but I don't find the answer to it... Why when using a bitrate of 6000Kbps I get almost 9fps on 2nd pass, but if I rise the bitrate to 25000Kbps (keeping all the other settings the same) then my 2nd pass encoding speed will decrease to 2fps?

I thought that with lower bitrate the speed would be less due to "what should I keep and what should I exclude" calculations whereas high bitrate should be faster because the optimizations would be less, but it seems that's not true.

Could anybody please give me an explanation?

Thanks

[nurbs]

The time CABAC takes is proportional to the bitrate and it's a significant fraction of encoding time to begin with. Other stuff like analysis options depend mainly on resolution and will take roughly the same time with the same settings regardless of bitrate AFAIK.

[Dark Shikari]

Fewer skips, more CABAC. This is true of any encoder, doubly so if you're using trellis.

[pepelugil]

OK, now it's clear.

Thanks nurbs & Dark Shikari