Looks like you forgot to link to the Scala.js blog post. FWIW, as a Scala.js user the combination of Bucklescript + Reason ML looks pretty compelling: static types and lightning fast build times.
Haven't used Clojurescript but I'd imagine the code-change-reload cycle is quite fast compared to Scala.js, which while absolutely excellent in many ways, suffers from the dog slow compiler that is scalac (Scala collections blowing up generated binary size is the other drawback that comes to mind).
I work on a 20K line enterprise CLJS app. Due to incremental compilation where only recent changes need to recompile during development, each code change (meaning, saving some source files) is on average 1 second of recompilation. With the Figwheel tool, there is no manual reload required, as it automatically refreshes the browser to reflect your most recent change.
> each code change (meaning, saving some source files) is on average 1 second of recompilation.
That's actually fairly slow, I'd expect pretty much instantaneous (hot reload is nice though). The overhead of Bucklescript, for example, is at most in the 10s of ms for code changes (saw a 2017 Strange Loop talk where the presenter claimed 200ms to compile the entire project).
Scala.js is around 2 seconds for shallow changes (affecting only one source file), which over time is a productivity drain, those seconds add up when doing front end work.
Haven't used Clojurescript but I'd imagine the code-change-reload cycle is quite fast compared to Scala.js, which while absolutely excellent in many ways, suffers from the dog slow compiler that is scalac (Scala collections blowing up generated binary size is the other drawback that comes to mind).