time java -cp cljs.jar:src clojure.main build.clj
For comparison, on my MacBook Air (11-inch, Mid 2013, 1.4 GHz Intel Core i5, 8GB RAM), while lots of software was running, so not a complete fair test (the Pis were booted up into the console, not even running X) this takes 5.75 seconds.
Considering the difference in specifications of these machines, the Raspberry Pi 2 is remarkably fast. I could even load up and play with Sonic Pi pretty quickly (on the original Model B that took long enough to make a cup of tea).
Looks like I'm going to be drinking less tea in the future.