If you are using a current version of your C compiler and can consistently reproduce those numbers just by adding something like -O3, you should probably file a bug report. Optimizers can go wrong and sometimes pessimize things a bit, but an almost 50% slowdown from enabling optimizations would be treated as an important bug to fix. (Though people are saying that significant amounts of time in the benchmark are spent in random number generation and output, so maybe try to find out first if the problem lies in one of those.)