"Play Rate" below 1 and "Force Fixed Frame Interval Playback" causes choppy renders.
This was explained by Max on Answerhub to the user, but this seemed like something that should be bugged and looked into.
1. Create a scene with a sphere mesh
2. Create a sequence with a track for the mesh
3. Create a simple translation animation for the mesh
4. Ensure that "Force Fixed Frame Interval Playback" is enabled under General Options
5. Create a Play Rate track with a key of "1" at the start and "0.5" at 2.5 seconds
6. Render a movie sequence out
Result: When it hits the 2.5 second mark, you'll notice that the playback becomes very choppy. If you disable "Force Fixed Frame Interval Playback" and re-render, it'll be smooth.
Expected: Play Rate Track doesn't conflict with "Force Fixed Frame Interval Playback"
Head over to the existing AnswerHub thread and let us know what's up.