Because a majority of the time, the idiomatic Clojure will be fast enough. I wouldn't consider Clojure to be a "slow" language, most of the time it'll perform

Because a majority of the time, the idiomatic Clojure will be fast enough. I wouldn't consider Clojure to be a "slow" language, most of the time it'll perform pretty well, and has really great utilities for doing multithreaded code (which can also increase performance). By using Java, you sacrifice a lot of the cool lispey features that you get with Clojure, and as long as it's possible to optimize Clojure in the parts that need to be optimized, then I think the language is worth it; depending on the context, you might even be able to write a macro to do these optimizations automatically while keeping the code somewhat idiomatic (though of course one needs to be careful with macros).