There is no such thing as "idle time" The CPU is guessing it based on the number of context switches.
Incedentally, CPU usage and memory are not related. If you need to swap something into or out of memory, the CPU moves onto the next proccess while waiting for the disk to get done it's job. Virual memory causes no CPU usage. (The reason why it seems slow though is because you;re waiting for the hard disk, but the processes that can run are running)