WJ wrote:

> (time (set! bigfib (fib 50000)))
>     562 ms real time
>     516 ms cpu time (500 user, 16 system)

This Ruby version runs in about 0.6 seconds:

def fib n
  a,b = 0,1
  2.upto(n){
    a,b = b, a+b
  }
  b
end