GHC performance mysteries

test.hs

import Control.Monad (MonadPlus(mzero))
import Control.Monad.ListT (ListT)
import Data.Copointed
import Data.List.Class
import Prelude hiding (enumFromTo)

-- This is exactly the same as Data.List.Class.enumFromTo
pastedEnumFromTo :: (List l, Enum a) => a -> a -> l a
pastedEnumFromTo from to
    | fromEnum from > fromEnum to = mzero
    | otherwise = cons from (pastedEnumFromTo (succ from) to)

nums :: [Int]
--nums :: ListT ((->) ()) Int
nums = enumFromTo 0 10000000
--nums = pastedEnumFromTo 0 10000000

main = print $ div (copoint (foldlL (+) 0 nums)) (copoint (lengthL nums))