[plt-scheme] Dr.Scheme freezes on (fib 100) recursion

my son and i coded a program to compute fibonacci sequences. it works fine for low numbers, but when you try to do something higher like (fib 100) Dr.Scheme freaks out and freezes up. we're wondering what's going on? is there a reason for this. . . .

(define (fib n)
  (if (< n 3)
      1
  (+ (fib (- n 1)) (fib (- n 2)))))