View source for VSG:Usage:Codec:SlowPlayback

You can speed up the H.264/MPEG-4 AVC playback by disabling loop filter for H.264 decoding. To do this go to '''Tools -> Preferences''' 
(set Show Settings to All) and '''Input / Codecs -> Other codecs -> FFmpeg''' and in the drop-down box for '''Skip the loop filter for 
H.264 decoding''' change it to '''All'''. Remember to press '''Save''' to save VLC settings and restart VLC after that to make sure
changes are enabled.

You can also try GPU based video decoding (works on NVIDIA Geforce 9xx0 and newer cards also on ATI Radeon HD 5xx0 and newer ones). You
can enable it from '''Tools -> Preferences''' and '''Input & Codecs''' and tick '''Use GPU acceleration'''. Remember to press '''Save''' 
to save VLC settings and restart VLC after that to make sure changes are enabled.