Alpha 15 is, for the physics part, all about performance. Has been the topic of various threads recently actually.
With a new build, I just made a test running Earth+5000 moons and it shows 97%-99% cpu utilization here.
The physics coming in alpha 15 is running only on cpu, in managed code, but it is faster than the previous combined opencl+cpu. This is essentially due to legacy design... how the data was being passed around.
In following releases, post alpha 15, native cpu and gpu computing will be enabled again, and this should be very much faster still.