But that was the point - you can write it cleanly in ClojureScript, experiment, find bottlenecks, then drop down to Javascript for the tight parts, and clean it

But that was the point - you can write it cleanly in ClojureScript, experiment, find bottlenecks, then drop down to Javascript for the tight parts, and clean it up (beautifully) with macros. There's obviously a tradeoff here with immutability/correctness/speed, but when you need to iterate over thousands (or hundreds of thousands) of items, you can get 'native' speed when you need it.