threadedにすると何故か遅い

-threadedでコンパイルすると何故か遅くなる。
2通りの実装書いたけど、どっちもだいたい同じパフォーマンスと傾向。

{-# LANGUAGE OverloadedStrings #-}

import           Control.Applicative
import           Control.Concurrent       (threadDelay)
import           Control.Concurrent.Async (async, race_, wait)
import           Control.Monad
import qualified Data.ByteString          as S
import           Data.ByteString.Char8 ()
import           Data.Conduit
import           Data.Conduit.Network
import           Network
import           System.Environment
import System.IO

main :: IO ()
main = withSocketsDo $ do
  [tn, rn] <- map read <$> getArgs
  server `race_` do
    threadDelay 1000
    mapM_ wait =<< replicateM tn (async $ client rn)

{-
server :: IO ()
server = runTCPServer (ServerSettings 12345 "*") $ \src sink -> src $$ sink

client :: Int -> IO ()
client n = runTCPClient (ClientSettings 12345 "localhost") $ \src sink ->
  src $$ cond =$ sink
  where
    cond = replicateM_ n $ do
      yield "hello"
      void await
-}

server :: IO ()
server = do
  ssock <- listenOn $ PortNumber 12345
  forever $ do
    (h, _, _) <- accept ssock
    let go = do
          bs <- S.hGetSome h 1024
          when (not $ S.null bs) $ do
            S.hPut h bs
            hFlush h
            go
    void $ async go

client :: Int -> IO ()
client n = do
  h <- connectTo "localhost" (PortNumber 12345)
  replicateM_ n $ do
    S.hPut h "hello"
    hFlush h
    _ <- S.hGetSome h 1024
    return ()
  hClose h