With h264 mkv files: the audio plays right away and the video remains black for quite some time. I often have to seek back to the beginning with the arrow keys to get the video to appear.
This happens only the first time I play a video after several hours of leaving MC alone. After that, video startup is immediate. I don't think it's a disk spinning up issue since the audio plays.
Builds before about 180 or so were fine. I may be mistaken it seemed to correlate with a new version of LAV filters.