Design it, Drive it : Speedboats has been update from Unity 5.3 to 5.5 with this patch, paving the way for Vive support to be added soon.
While updating, it was noticed that the water reflections system was doing work on more cameras than it should have been. This has had a significant impact on rendering performance all along. This patch removes the extra computations so now the water reflection/refraction rendering effects are only applied to one camera.
This has a couple of effects: The inside of the windshield on the vee is now more reflective. In fact it was reflecting the bright floor so much that it almost looked like the windshield was hazy or dirty. The floor has been darkened which cancels out some of this, leaving the windshield looking pretty clean and clear.
The other effect of this patch is a significant increase in frame rate. Stuttering in the display (on the development hardware at least) is now reduced noticeably. While not perfect, it's quite a bit better than before. Virtual reality rendering performance also is improved and is a bit smoother.
There was another bug that could occasionally cause the simulator to crash after some time while changing scenes or out driving. This one appears to have been fixed in an unannounced patch that was snuck in yesterday, and remains in place for this patch.