[astrange]

IIRC x264 does have a lookahead motion estimation that can run on the GPU, but I wasn't sure I could explain this properly.

That said, I disagree because while motion estimation is parallel, motion coding is not because it has to be "rate distortion optimal" (depending on your quality/speed tradeoff.) So finding the best motion for a block depends on what the entropy coder state was after the last block, because it can save a lot of bits by coding inaccurate/biased motion.

That's why x264 and ffmpeg use per-frame CPU threading instead (which I wrote the decoding side of) because the entropy coder resets across frames.