WebM video transcodes include a cue index of keyframe locations, by default placed at the end of the file by ffmpeg. Because it's at the end, if the user tries to seek within the file with the scrubber bar the browser will have to launch a sub-range request to go fetch the cues before it's able to find the position to seek to. This is an extra network round trip and can be slow.

There should be tools out there which can rewrite the file to move the cues to the beginning, which means we've already read the cues at the beginning of the file and seeking can happen at any time.