Performance improvements for 0.21?

[Blackstar]

One weird thing I've found is that having multiple things docked together really kills the framerate. My space station, consisting of 27 distinct "vessels" (many of them just orange fuel tanks with a docking port stuck on) caused massive slowdown to approach. After some launch pad testing, I loaded all the parts into the VAB, pulled out the docking ports, and attached all the pieces directly together. I then replaced the station in my persistence file with that version.

Same exact station, but with the parts directly joined instead of docked together -- smooth as silk.

Interestingly, this particular slowdown I don't think was physics-related, because it persisted even while in time acceleration.