[Haskell-beginners] parallel program in haskell in 5 steps

In step 4 of Haskell in 5 Steps [ http://haskell.org/haskellwiki/Haskell_in_5_steps], a parallel program is given. I changed it very slightly so it would run a long time (see below). It compiles and runs but my CPU meter is barely above 50%. I have a dual core processor. What in the world would keep this program from completely saturating the CPU? import Control.Parallel main = a `par` b `pseq` print (a + b) where a = ack 4 10 b = ack 4 10 fac 0 = 1 fac n = n * fac (n-1) ack 0 n = n+1 ack m 0 = ack (m-1) 1 ack m n = ack (m-1) (ack m (n-1)) fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2)