All arithmetic operations on these boxed objects are
    significantly slower than on their primitive counterparts.
    This implementation also creates an unnecessary intermediate sequence
    (the result of the map), rather than just summing
    the numbers directly.

Clojure's Reducers framework might address the described issues in a future when, in Rich's words, "those IFn.LLL, DDD etc primitive-taking function interfaces spring to life". For now, they only solve the intermediate-collections part of the problem.