The performance of mapM appears to be supralinear in the length of the list it is mapping on.  Does it need to be this way?  As a comparison, both mapM_ and map are linear in the length of the list. To wit: travis@PW:~/Documents/insurer$ ghci GHCi, version 7.0.3: http://www.haskell.org/ghc/  :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :set +s Prelude> :set +t Prelude> :m Data.List Data.Maybe Prelude Data.List Data.Maybe> foldl' (+) 0 $ fromJust $ mapM (\x -> Just x) [1..500000] 125000250000 it :: Integer (2.23 secs, 112875864 bytes) Prelude Data.List Data.Maybe> foldl' (+) 0 $ fromJust $ mapM (\x -> Just x) [1..1000000] 500000500000 it :: Integer (6.86 secs, 214002204 bytes) Prelude Data.List Data.Maybe> foldl' (+) 0 $ fromJust $ mapM (\x -> Just x) [1..2000000] 2000001000000 it :: Integer (24.39 secs, 429299748 bytes) Prelude Data.List Data.Maybe> foldl' (+) 0 $ map (\x -> x - 1) [1..1000000] 499999500000 it :: Integer (0.77 secs, 171213436 bytes) Prelude Data.List Data.Maybe> foldl' (+) 0 $ map (\x -> x - 1) [1..10000000] 49999995000000 it :: Integer (7.42 secs, 1723399472 bytes) Prelude Data.List Data.Maybe> foldl' (+) 0 $ map (\x -> x - 1) [1..40000000] 799999980000000 it :: Integer (30.46 secs, 6894835952 bytes) Prelude Data.List Data.Maybe> mapM_ (\x -> Just x) [1..1000000] Just () it :: Maybe () (0.42 secs, 82761248 bytes) Prelude Data.List Data.Maybe> mapM_ (\x -> Just x) [1..10000000] Just () it :: Maybe () (3.87 secs, 808012660 bytes) Prelude Data.List Data.Maybe> mapM_ (\x -> Just x) [1..100000000] Just () it :: Maybe () (38.40 secs, 8054769564 bytes) Prelude Data.List Data.Maybe>