Great work as always with this new prerelease ! :)
Just one thing: I'm having an small issue with the hotspots orientation in my panos (Firefox39.0.3 and IE11) with pr3.
I've updated the "Immersarium" web experience (http://www.noacco.net/immersarium) to pr3, and when viewing in VR mode, everything is fine. But when I toggle back to "normal mode", some hotspots don't have the correct rotation anymore.

You can see it live when going to http://www.noacco.net/immersarium/discoverymountain, and choosing "Enter the Stellar Way".
The rest works really well: better performance on mobile devices, smoother pov rotations: again great work !