I am pretty convinced that we won’t see a background thread video export any time soon. However if I understood Timo correctly replace audio is in the plan. When implemented it will be fast but will still create to large video files for many use cases.
QT7 is depreciated and won’t work at all on new Mac OSes. I have since a while back transitioned to MediaER toolkit a great ffmpeg frontend specifically created to deal with this for us audio folks.