Random performance (string concatenation) related discovery:

> (let [num 10000000]
    (time
     (dotimes [i num]
       (str "abcd" 42 "jadda" \newline)))
    (time
     (dotimes [i num]
       (str "abcd" 42 "jadda\n"))))
"Elapsed time: 2185.083983 msecs"
"Elapsed time: 1577.440842 msecs"

..perhaps obvious, but still some easy wins.