Just with mkv and mp4, you can't say for sure what video codec and profile is used inside. N9 supports h.264 baseband and most of those HD mkv videos are High or Main profile which isn't supported by N9 DSP decoder, this results in lags or playback not supported at all. download those divx/xvid (avi), I assure you that they play perfectly on N9 and takes a lot less space.