Unable to play MPEG4

[diet]

Using appimages 6.3.0 and 6.4.0, video playback is extremely jerky with files of type MPEG4 at 24 fps. The same files play well using the default video player. (The problem does not exist with H.264 at 23.98 or 50 fps.) Running Fedora 31 / GNOME.