Most games that run on mobile devices are really, really simple. Take angry bird for example: It is a couple of circles (birds) and rectangle (obstacles). Precision is not in focus, and thus you can take some shortcuts.
Try the following in Settings.cs:
- Disable CCD
- Lower the number of iterations (VelocityIterations and PositionIterations)
- Try setting ConserveMemory to true - it is untested, so beware.
You also need to test performance in release mode and remember to set Setting.EnableDiagnostics to false when you release.
The only other thing you can do is to minimize the number of active bodies.