I think the brouhaha over Mandreel vs Emscripten glosses over a deeper issue, which is, the vast majority of Web apps aren't C++ compiled to asmjs, and performance on asmjs is not going to be a good proxy for general web performance.

"Real apps" using idiomatic JS should be used as benchmarks, and those apps should approximate apps in widespread deployment. For example, how fast can a reactjs re-render for a complex page run?