Gildaniel, Sweet Home 3D JS is slow when changing levels because it recomputes all door and window holes in the walls, and the computing of these holes is much slower with JavaScript than with Java.
But there's surely some place to optimize this process, even if it's not simple.

I released version 1.6 of the plug-in today to include the improvements available in the new Sweet Home 3D JS 6.4 Viewer, like a better management of superimposed transparent objects and a fix for a bug which prevented to pinch under iOS.
I also included some comments in the [font=courier new]viewHome.html[/font] exported file to explain how to simply display the viewer in full screen.