At the moment HaRe has -O0 set for the library. Here are some timing differences on my machine with optimisation on and off With normal optimisation, we get $ rm -fr dist-newstyle/ $ cabal new-configure --enable-test $ time cabal new-build real 6m54.778s user 7m3.156s sys 0m4.504s With -O0, the timing for the same commands is real 0m36.427s user 0m41.448s sys 0m2.544s So optimisation slows it down from 36 seconds to 7 minutes. Alan