I am experiencing the same bug, where Set Rate does nothing.
Regarding the frame rate issue, I have been having this problem for a while now, and unfortunately there hasn’t been any resolution yet. I posted this for version 4.8, before Sequencer existed:
Basically, my workaround proposal is to artficially lock the export frame rate, and set the video playback rate to match.
So, if I’m getting around 2-3 fps on export, I’ll lock it to something lower, like 1 fps, and set my video playback rate to 1/30, or whatever the frame rate of the source video is.
While this might not give me perfect sync, it should be close enough, and it is a far more desirable situation than what exists - pure guesswork about the output frame rate (which can change depending on the camera angle, and constantly plugging in Set Rate values, hoping that one works.
For someone like me, who always needs high res exports and video in the scene, the current set up is a nightmare to deal with.