Ticket #5272 (closed bug: fixed)
The code generated by ghc 7.0.4 is significantly slower than the one for ghc 6.12.4 for one of our large programs.
For a typical benchmark, with ghc-6.12.4 the compiled code ran in 110s and with ghc-7.0.4 it runs in 125s. This is 14% slower. The program was compiled with the same flags in both cases, notably -O2.
It's difficult to pin down the difference, because when compiled for profiling they run at about the same speed. And as usual the profiling information from ghc is wildly inaccurate (with both versions, but in different ways).