Writing to stderr is 7x slower than writing to stdout

For some reason printing to stderr is significantly slower than printing to stdout.

I came across this because I realized that simple !QuickCheck properties are unnecessarily slow. !QuickCheck writes a lot of output to stderr, overriding old output with \b.

Here are some stats:

GHC 7.6.1 / xterm / stdout:

main = sequence_ . replicate 10000 $ hPutStr stdout "foobarbaz" >> hFlush stdout

$ ghc main.hs && time ./main
...
real    0m0.070s
user    0m0.016s
sys     0m0.032s

GHC 7.6.1 / xterm / stderr:

main = sequence_ . replicate 10000 $ hPutStr stderr "foobarbaz" >> hFlush stderr

$ ghc main.hs && time ./main
...
real    0m0.538s
user    0m0.112s
sys     0m0.228s

If I do the same thing in C, then there is no performance difference.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
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