[gTen]

A class 2 card should be plenty to play 480p..most of the time a class 2 card would go at same speed a class 6 or even 10 card goes for consecutive reads..only time it starts to vary is when the card begins to fragment.

That said Rock Player uses "Software Decode" rather then hardware decode, hardware decode is a lot more efficient, in software decode it puts all the pressure on the cpu/gpu..even though the movie is 480p thats only the resolution there are other things that can effect it such as filters and etc...its probably likely that the snapdragon cpu/gpu cant keep up with the processing power required.

As for subs..there are different formats, such as .sub, .srt, .ass that are embedded into the mkv..make sure that the player supports the embedded sub format.