I don’t know where the overhead on mobile is coming from. I’ve seen other emscripten engines run pretty decent on mobile html5 too. Also browsers are now using webgl and accelerated views.
The problem is that it lags even on an iPhone 6 with a bare bone example and a tiny bunch of go.
A bit of love in this area would be greatly appreciated as html5 mobile is often a requirement when dealing with game portals.