Actually, any time you are doing two things at once, you will notice a performance increase with a DP system over a SP system. Since a machine is almost always going to be doing more than one thing (executing more than one process) while someone is using it, you can see a performance benefit. If you are a multitasker, and have several different apps running at the same time, you will notice a big difference on an MP system over a uniprocessor system. This is the premise behind why Intel added HyperThreading to their processors. Testing a single application's performance should never decrease it over single processor, and should increase it if it has been written as a multithreaded application.