Could you try IOUArray for completeness too? (An IOUArray is the unboxed version of IOArray, it can be found in Data.Array.IO).
It fits in as the fastest: IOUnboxedMutArray 0.48u 0.04s 0:00.58 89.6%
NormalArray 1.65u 0.20s 0:01.89 97.8% NormalArrayReplace 2.40u 0.08s 0:02.56 96.8% UnboxedArray 0.80u 0.04s 0:00.87 96.5% UnboxedArrayReplace 1.83u 0.07s 0:01.99 95.4% IOMutArray 0.60u 0.03s 0:01.09 57.7%
You could try testing DiffArray (Data.Array.Diff) which is optimised for in-place updates, and should show a bigger difference between the normal and 'replace' versions. It might be nearly as fast as IOArray (I don't think we've ever benchmarked it), and it doesn't need to be in the IO monad.
DiffArray seems to be broken :). Either that or I'm using it incorrectly. I've attached the relevant code, but when I don't reverse the array everything works fine; when I reverse it the program doesn't (seem to) halt. module Main where import Data.Array.IO import Data.Array.Diff testArray :: IOToDiffArray IOArray Int Int testArray = array (0,50000) [(i, (19*i+23) `mod` 911) | i <- [0..50000]] reverseArray :: IOToDiffArray IOArray Int Int -> IOToDiffArray IOArray Int Int reverseArray arr = arr // [(50000-i, arr!i) | i <- [0..50000]] sumArrayMod :: IOToDiffArray IOArray Int Int -> Int sumArrayMod arr = sumArrayMod' low 0 where sumArrayMod' pos sum | pos > high = sum | otherwise = sumArrayMod' (pos+1) ((sum + arr!pos) `mod` 911) (low,high) = bounds arr main = print $ sumArrayMod $reverseArray testArray