To give you a little bit of technical information about what's going on here.
When the impacts happen, a lot of extra bodies are instantly created, they're in very close proximity to the parent body, and because of this, the Physics engine has to step in small increments to make sure that collisions are not missed. It's a whole cloud of colliders very close to a larger one. As well, it needs to calculate the gravitational influence of the parent on all the bodies that are spawned.
As well, it tends to create a lot of sub impacts and every impact is carrying mass and requires a transfer from one body to the other, there's also sound FX, visual FX and other systems that come in to play. Long story short it's a whole flurry of activity in a short time and it will temporarily bog things down.
One of the current things we're working on is improving how our impact events work, so we're not sending a ton of events for a single collision (they've been sent every step of the collision in the past), Instead we now send one event and keep it updated with relevant information about the ongoing event. This allows us to budget the fragments, particles and FX a lot better then we ever could before.
We're also making changes to how the physics engine steps, and what it sees as important enough to globally slow down things for increased accuracy. It's now giving lower importance to things like the particles so they don't cause such a problem. As well, we discovered a bit of a bug with the automatic performance controls, they were relying on information that wasn't being reported, so it was actually increasing the requested step length when performance got bad instead of easing up on things.
All of these changes require a lot of testing, and it will be a little while yet till they all get incorporated into a release. Getting the game running smoothly and reliably is a priority for us.