Too slow H.264/MPEG-4 AVC playback
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.
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.