Intel Mac using Windows XP, Vista, and Mac OSX. XP seems to be 5 to 15% faster than OS X even with Boot Camp limitations. (via)
Update: There's probably no conspiracy against the Mac -- which admittedly is a rapidly "evolving" platform. The current Mac version maybe a bit slower because code generated by Apple's Xcode/gcc is not quite as fast as the Windows compiles and/or the AE team has had a lot longer time to optimize things for Windows, which has not undergone so many processor and OS changes as the Mac. Happily, the new Intel Mac platform should allow Intel-specific optimizations to help both platforms.
Update 2: General Specialist adds, "Still, there's one BIG reason why Windows on Mac hardware isn't the ultimate solution; that the Apple hardware only gives the 32-bit versions of XP and Vista access to 2 GB of RAM on Macs. The solution is to use Vista64 or XPx64."
Update 3: Ken Broomfield thinks (AE-List, 16 Aug 2007), "The performance difference here is probably due to Carbon and the MacOS's need to support antique programming interfaces (APIs) on a modern OS, which introduces all kinds of overhead."