With MP4 I used this command

ffmpeg -i INPUT.mp4 -vf ass=SUBS.ass -c:v libx264 -preset ultrafast -crf 22 -c:a copy OUTPUT.mp4

which has helped to increase the speed.

But there are some problems with WebM format, I tried to use this command which didn't help

ffmpeg -i INPUT.mp4 -vf ass=SUBS.ass -c:v vp9 -crt 22 -c:a copy OUTPUT.mp4

1 Answer 1


Error occurs. Something like this:

[webm @ 0x5] Only VP8 or VP9 or AV1 video and Vorbis or Opus audio and WebVTT subtitles are supported for WebM.

Do what they ask:

ffmpeg -i in.mp4 -map 0 -c:v libvpx-vp9 -crf 18 -b:v 0 -c:a libopus -b:a 213k -ac 2 -c:s webvtt -y out.webm

replace options with yours.

[update] For increasing speed better use MKV. Fastest way:

ffmpeg -i in.mp4 -i subs.ass -map 0 -map 1 -c copy out.mkv

or MP4:

ffempg -i in.mp4 -i subs.ass -map 0 -map 1 -c copy -c:s mov_text out.mp4
  • Thanks Баяр, at least it works now, but there is no increase in speed, so if anyone could add something...
    – R St
    Oct 19, 2020 at 14:48
  • Unfortunately, you can't increase speed, because you have to reencode every track in input files. Oct 19, 2020 at 15:15
  • But I could reencode in this case: superuser.com/questions/1577401/… Why can't I do something like that with WEBM?
    – R St
    Oct 20, 2020 at 19:31
  • You must reencode all tracks to another format to fit into WEBM. You can't copy some tracks for increasing speed. You can try to use hardware codec. If u have intel gpu use "-c:v vp9_qsv". Or do not use WEBM, use MP4 or MKV. Oct 21, 2020 at 5:03
  • 1
    "vp9_qsv" for windows only encoder Oct 27, 2020 at 4:31

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