"the debugging story for generators is somewhat bad, especially because of the missing stack traces for thrown errors. Fortunately, there are solutions and workarounds, like those implemented by genny (obtrusive, reduces performance) and galaxy (unobtrusive, but requires native modules)."

From the post you brought up, http://spion.github.io/posts/analysis-generators-and-other-a... -- by the author of Genny. As I said, that's at least qualitatively the same as Q; quantitatively I haven't looked. At https://github.com/spion/genny there's the example line

    require('genny').longStackSupport = true

and "This results with CPU overhead of approximately 500% and memory overhead of approximately 40%. In the future, the overhead will probably be eliminated in node but not in browsers." The benchmark post says Catcher is worse.

The README for Q shows the same switch and says "This feature does come with somewhat-serious performance and memory overhead, however. If you're working with lots of promises, or trying to scale a server to many users, you should probably keep it off. But in development, go for it!"

I'm not a current user of any of these systems, but this is why I said the first-order cause of any performance difference appears to be that ClojureScript is not a library.