Can someone share some insight on why Flash is so much more performant at these types of operations? I recall not too long ago when the Javascript + Canvas demos of this type weren't even possible with a couple hundred particles, and this one does an outstanding job, but if you click on the Flash version, you can fill it so full of particles that the physics break, and there's little or no lag (on my system). Why is that, and can we ever expect Javascript + Canvas (or similar tools) to catch up to Flash?

I think (but have not verified this) it's mostly the rendering that makes the Canvas/JS so slow. If you'd turn off the rendernig in both versions you'd probably get similar rates.