Most of the speed improvements are thanks to the Eigen C++ template library for linear algebra. In particular, this enabled me to easily write SIMD code rather than using floats.
Other bits of interest are the realtime OSC server and an interface for the Control (OSC) app. Users with iOS or Android tablets can control the physical modelling with their fingers! I'm hoping to have a video in a few days, although I still need to find somebody with a video camera. Before you get too excited, I should add that this is a proof-of-concept of the communication between tablet and computer; I haven't put any thought towards the actual GUI or ease of control. My focus is on finishing my thesis, so if anybody is interested in making better interfaces, go nuts!
There's sure to be rough edges left in this release. I'm trying to force myself to accept "the perfect is the enemy of the good". I'm having huge problems writing my thesis because whenever I write down what I've done, I think of possible flaws, which immediately prompts me to spend a week or two performing experiments which justify what I've done (or else demonstrate a real problem, in which case I spend another week or two changing even more!).
With that in mind, I expect a 2.2 release to happen within the next month or two. Of course, if more people test the 2.0 version and report problems, I'll have more confidence (and more motivation!) to get the 2.2 release out there.