I don't know. Javascript is pretty slow as it is, and clojurescript is even slower. There is an inherent problem with compiling a dynamic typed language to another dynamic typed language...if your target language doesn't have the same semantics as your source, you end up having to embed a bunch of runtime code into your compiled output, or sacrifice compatibility. [0]

With a strong statically typed language like scalajs or elm, you have a lot more static reasoning that can be done at compile time, allowing you to omit a lot of runtime support.

[0] http://www.lihaoyi.com/post/FromfirstprinciplesWhyIbetonScal...