Bear in mind...
... that "a Linux slice with an effective 105MHz of processing capacity and just under 1GB of memory" is not the whole story. The VMs aren't limited to these figures, these are the averages if each VM was running flat out at the same time. The machine is very good at directing the CPU and memory towards the VMs that need it at each split-second, so whilst one's CPU might be idle whilst it does some I/O, another might have multiple cores running at serveral GHz, crunching some numbers.