This could be Windows-specific, since I only use VLC on Windows. If I'm playing an .mp4 file at a slowed down speed, using hotkeys to jump hangs for around five seconds before it starts playing again. This doesn't happen on all file formats. It's as though the speed factor causes it to miscalculate where to look in the file, so it has to spend time fishing around, finding its place again. Sounds like a bug to me.
Is VLC using all built-in codecs, etc., or is it using those of the OS? If the latter, is there a better codec available that doesn't have this problem? Or is it purely a VLC issue?