I just noticed that I/O performane on Windows without -threaded is pretty terrible. The following program: main = getConetnts >>= putStr tested with a 1MB file: $ rm cat.exe; ghc-7.0.1 --make -rtsopts cat.hs $ ./cat <1M >/dev/null +RTS -s <snip> Total time 0.28s ( 0.49s elapsed) <snip> But if we add -threaded: $ rm cat.exe; ghc-7.0.1 --make -rtsopts cat.hs -threaded $ ./cat <1M >/dev/null +RTS -s <snip> Total time 0.05s ( 0.07s elapsed) <snip> Several times better. I just made a tweak in my working tree to increase the I/O buffer sizes, and this helps quite a lot: $ rm cat.exe; ../ghc-testing/inplace/bin/ghc-stage2.exe --make -rtsopts cat.hs $ ./cat <1M >/dev/null +RTS -s <snip> Total time 0.06s ( 0.05s elapsed) <snip> $ rm cat.exe; ../ghc-testing/inplace/bin/ghc-stage2.exe --make -rtsopts cat.hs -threaded $ ./cat <1M >/dev/null +RTS -s <snip> Total time 0.05s ( 0.05s elapsed) <snip> Why is this? I'm not entirely sure, but I suspect it is the implementation of asynchronous I/O in the RTS that is only used on Windows without -threaded (rts/win32/AsyncIO.c etc.). Every I/O operation in the non-threaded RTS on Windows involves a couple of OS-thread switches. I'll get the buffer size fixes into 7.0.2. Cheers, Simon