(define* (bm thunk #:optional [repeats 1])
  (define (loop reps acc)
    (cond
     [(= 0 reps) acc]
     [else
      (let ([start-time (get-internal-real-time)])
        (thunk)
        (loop (- reps 1)
              (cons (- (get-internal-real-time) start-time) acc)))]))
  (exact->inexact
   (/
    (apply + (loop repeats '()))
    repeats)))

(bm (lambda () (+ 5 5)) 5000000))