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