> If I may ask you, do you think the fact that Windows 7 is handling parallelism > (multiple cores) better is a factoring much in the performance. Maybe the answer is > somewhere in JohnIV benchmarks but then I missed it.
The speedup is primarily because x64 has twice as many CPU registers as 32-bit and compilers can generate substantially better code in that situation. It's not related to the properties of any specific operating system since MAME sees a similar speedup ratio on all OSes.