I've wrote simple Haskell benchmark program, which populated primitive vector from vector package: import Data.Vector.Primitive.Mutable as P vectorBench :: Benchmark vectorBench = bgroup "vector" [ primitive ] where primitive = bgroup "primitive" [ write1M ] where write1M = bench "write1M" $ nfIO $ do v <- P.unsafeNew 1000000 void $ for [0..1000000 - 1] $ flip (P.unsafeWrite v) (1 :: Int) return () I use `unsafeNew` to skip memory initialization and `unsafeWrite` to skip boundary checks, I guess it's fastest possible way to write something to vector. My result was about 40 ms. I wrote similar program in Scala: for (_ <- 1 to 5) { val start = System.currentTimeMillis() val a = new Array[Long](1000000) for (i <- 0 until 1000000) { a(i) = 1L } val end = System.currentTimeMillis() println(s"${end - start} ms") } I skip neither boundary checks nor memory initialization, I also used generic array here (suitable for any types of objects, not just for primitive types), so I expected longer run time. But what I got was shocking: 7 ms 3 ms 2 ms 1 ms 2 ms This program runs 20-40 times faster than Haskell after warm-up. 20-40 times, Carl! Why is Haskell soo slooooow? How can it be? -- Sincerely, Stanislav Chernichkin.