As we all know the stopwatch might have issues in a multi-threaded async application that will be running on all cores.
On a multipro
Actually, according to the official documentation, post-Windows XP, it should be just fine to use StopWatch / QueryPerformanceCounter on multiprocessor systems. If the system does not support invariant TSC, QPC will automatically use a different timer strategy.
Do note the comment section, though, for virtualized systems. It seems that you have to be careful in that specific case.