Hector Martin

h.264 frames can depend on an infinite number of frames before them. A more common setting is 30. That means decoding frame 30 requires decoding frames 1-29 too. So you need 30 times the CPU power to decode any *random* arbitrary frame in the time it normally takes to decode one.

Try encoding your video on the left with GOP size 1 and I bet it'll seek as smoothly as the video on the right.

(It will also be much larger, but still smaller than the video on the right since h.264 is still going to be a better codec even in intra only mode)

ffmpeg.exe -i welcome.mp4 -vcodec libx264 -g 1 welcome_gop1.mp4 it seeks fast now, but now it's twice the size of the original Sorenson video (which I deleted off of the host, so I just re-encoded the already re-encoded mp4, whatever)

Change the CRF.

Also re-encoding in intra only mode a video with interframe artifacts *is* going to increase the size for the same quality. No, not "whatever".

If you had the original raw uncompressed video you could do a fair comparison, but now you're just stacking the cards against h.264 in multiple ways.

I'm not comparing h264's efficiency! I'm comparing the player software! The software on the left is defective! There are players that aren't F'd up!