[martinced]

The reason JavaScript is dog slow is single-threading. That's quite a huge handicap on today's hardware and it's only gonna get worse and worse until JavaScript engines becomes multi-threaded.

Besides that, I don't disagree with TFA when it says: "Garbage collection means cleaner APIs which can intelligently allocate their own buffers and can therefore relieve the user of what can be a tiresome or error-prone burden."

But that's certainly not an explanation as to why GC languages are not slow. (and I'm not saying GC languages are slow: all I'm saying is that saying that GC leads to cleaner code has nothing to do with performances of the runtime)