But it's no joke. Emscripten and Asm.js (the latter started as a Mozilla research project) have been making major strides since their introduction. The latest step up is an Emscripten revision that replaces the original compiler core with one derived from LLVM, a compiler technology framework used mainly for C/C++ but in theory capable of supporting many other languages.
According to Emscripten's developers, switching to LLVM gives Emscripten several advantages. For one, compilation is much quicker -- "often four times faster or more," they claim -- and requires far less memory. It also "avoids unpredictable pathological compiler slowdowns that the old compiler had." Most significant, the LLVM back end allows much tighter integration between Emscripten and LLVM.
The only downside of using LLVM is that it has to be recompiled specifically to work with Emscripten. A stock build of LLVM only gives you a subset of the available benefits, which you can get away with using if you have no other choice.