When viewing a library with many videos having video thumbnail enabled leads to a DOS. Even worse is that when the first requests fails, seahub tries it over an over again. I also get the impression ffmpeg fetches the whole video to generate the thumbnail. I’m not a video export but it should be enough to get a portion of the file to extract the video thumbnail.
All in all the only solution I see is to optimize the feature (read as little of the file as possible) and to move it in a background job with a configurable amount of simultaneous ffmpeg processes.