Some OSes have built into them the ability to do several things at once. This is called multitasking, and has been in use since the late sixties / early seventies. Since this ability is built into the software, the overall system will be slower running two things at once than it will be running just one thing. A system may have more than one processor built into it though, and such a system will be capable of running multiple things at once with less of a performance hit.