We're working on some major changes for performance. The first parts are going in with Alpha 13, that focuses on the performance of the integrator itself. Which is how fast and how accurately it can advance time in a simulation.
We're also working on the second portion, which is ensuring we maintain a good frame rate, even when the physics engine takes a long time to solve. That's being worked on currently and may come in Alpha 14.
Long term, we're looking at some structural changes that will free the rendering and the physics from executing sequentially. That's a bit more of an undertaking though, and may take a while. (Several updates)
TLDR : Performance is really important to us, and we have a multi-step plan to improve it over time during the Alpha phase of development.