I am streaming an ogg video from the server and when I do so my gmae slows from around 60-70fps to around 20-30fps.(even with the movie not actually displayed but just playing.. eg myMovieTexture.isPlaying)
There are no additional lines of code being run during the video so its just the streaming and playing thats making the game take such a hit.
Does this sound correct? Also my movie is set to looping so surely if unity were sensible it would only download it once and then keep playing the downloaded data right??
As long as a Movie as playing, it will use CPU cycles, whether it is visible or not. Otherwise, the movie playback would have to jump to different positions when it becomes visible again, which our movie class cannot currently do (and also it requires decoding all the frames from the last keyframe in the movie to the current position, so it could cause jerkiness whenever a movie becomes visible).
However, you could try making the movie resolution smaller to reduce the impact of movie playback, or experiment with the compression settings. Maybe you will find a setup which is satisfactory for your use.