1. What browsers and device is this on?
2. This happens when the game is CPU limited. In this case lowering the graphics will not help and there isn't really much that can be done.
3. Is this in the Android app? I think it's the same issue as mentioned below
4. The next major update will contain some pretty big CPU performance optimizations, but this won't help in cases when you are GPU limited. I think large tracks might be more GPU limited so it might be harder to optimize.