MLton TimeLimit

Guess what this prints, and after how long:


val fastTime = Time.fromSeconds 2
val slowTime = Time.fromSeconds 6

fun fib 0 = 1
  | fib 1 = 1
  | fib n = fib (n - 1) + fib (n - 2)

fun bigfib () = fib 10000000

fun slow () = TimeLimit.timeLimit slowTime bigfib ()
    handle TimeLimit.TimeOut =>
           let in
              print "slow time out"
            ; ~1
           end

val _ = TimeLimit.timeLimit fastTime slow ()
    handle TimeLimit.TimeOut =>
           let in
              print "fast time out"
            ; ~2
           end

The rather unintuitive answer is "slow time out" after 6 seconds.
What is going on here?  I guess nesting of TimeLimits is dangerous.