Ok, so it is limited by hardware then.

Thanks!

I'm running latest 3.53 build, and run Tvheadend according to these instructions:
http://www.xbmc4xbox.org.uk/wiki/HTS_Tvheadend

It works fine with SD-channels, but HD-channels (720 and 1080) completely freeze the Xbox. I use DVD-Player since MPlayer does not work with Tvheadend at all.

Any ideas?