I am happy to say that our fears were unfounded. Performance, on the desktop, seems comparable to the current JavaSE port - and in some cases it is even better. The true test of performance, however, is on mobile devices. On newer devices (e.g. iPhone 6, iPad Air 2) the port is quite responsive; Animations and transitions are smooth - though not quite as smooth as in the iOS port. On the previous generation of devices (e.g. Nexus 5, Nexus 7), I would describe the port as “usable”, but animations and transitions are a little jerky. As we move into even older devices, the apps remain usable, but the slow responsiveness, animations, and transitions become more noticeable.
So… at present, your apps should work very well in desktop browsers and quite well in new mobile devices.