Bare Feats, a popular site devoted to Mac vs PC benchmarks,
benchmarked Doom 3 on the Mac to investigate what are the issues with Mac's slower performance in Doom 3 compared to the PC. They even contacted the developers responsible for the port who explained that it indeed has to do with Mac's architecture, gcc's slower optimizations and the OS X itself. From the article: "
PowerPC architectural differences, including a much higher penalty for float to int conversion on the PPC. This is a penalty on all games ported to the Mac, and can't be easily fixed."