This one drove me nuts for a while, watching 720p content over 54Mbit/s wi-fi should work fine but with the “out of the box” settings HD moves were stuttering terribly for me.
As an additional niggle, embedded subtitles were appearing but they were enormous.
By default MythTV uses mplayer for mkv, avi and mpeg files. To resolve the stutter and subtitle issue open the MythTV frontend and go to “Utilities / Setup” -> “Setup” -> “Media Settings” -> “Video Settings” -> “Player Settings” and set the “Default Video Player” to:
mplayer -fs -zoom -quiet -vo xv -subfont-text-scale 3 -cache 8912 -cache-min 4 -lavdopts threads=2:fast:skiploopfilter=all -sws 0 %s
You may find it easier to connect to the mythconverg database and set the VideoDefaultPlayer to the value above for the host machine on wireless.