I've made some more tests.
I ripped one of the DVDs to the PC hard disk, opened the .ifo and got the same problems (slow video + no sound)
If i open the .vob files they play ok (but, of course, no language change is possible)
Copied the ripped DVD to the Tvix HD. It played well opening the .ifo
I've got identical results with firm 1.2.23 and 1.2.27.
i did more tests with some HD movies in the PC HD, and all of them worked very well. (h264 mkv, from 1.5 GB to 7GB, 720p & 1080p)
i'm a bit lost

If it was a network problem why i can see movies with much more MB that the DVDs? And, why i can see the .vob ¿?
Maybe this should be moved to another thread?
** Edit: Forgot to say that i tried with another DVD drive, and got same results **