Looks like a poorly supported codec (you can check more information by pressing 'Report' button after video playback attempt in GUI frontend). MPlayer uses a lot of hardware specific optimizations to be fast. It has excellent supports for almost all codecs on normal desktop PC's (using Intel or AMD x86 cpu) as it has optimized code for most of them tuned during a long time by many people. Internet Tablets use ARM cpu. This different cpu architecture requires its own optimizations, so many less popular codecs don't have ARM optimized version and have to use generic code and perform poorly. The best supported codec for video is mpeg4 (divx/xvid) and its variants such as flv. It is also highly recommended to use mp3 audio. Other audio codecs may introduce serious performance problems.