[Zak]

Clojure can be exactly as fast as Java within a function when type hints are used to avoid reflection. Calling Clojure functions has a little overhead relative to calling Java methods.

That said, an idiomatic system in Clojure will likely have a very different design than it would in Groovy or Java. Using Clojure's built-in immutable data structures, it's cheap in terms of CPU and memory to make a large number of modified copies or to concurrently access data compared to the standard collections in Java and Groovy. Single-threaded reading and writing are, however somewhat more expensive.

My prediction is that it's easier to get good performance out of a system with a lot of concurrency using Clojure than it is with Groovy or Java, though with careful design, it's possible to get better performance with Java.