I have several 720p and 1080p .mkv movies. I tried to play those on Tegra 2 tablet (Acer Iconia 500) using MX Video Player but the picture lags on both 720p and 1080p.
What hardware is theoretically enough to play at least 720p HD movies?
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Both Tegra 2 and Tegra 3 NVidia chipsets are able to manage 720p/1080p HD movies. In fact I own an Asus Transfomer TF101 (Tegra 2) and I was able to play successfully lag-free various 720p MKV movies.
At the end MKV is only a container and, as suggested by @Chahk in comments, it could be that you are trying to view movies with an excessive bit rate. Try to recode a short 720p video reducing the bit-rate and check if you have better results.