To put a slightly different spin on qwertox's reply: What matters is that the OS can guarantee how long it will take for, say, a high-priority task to become active after it is activated. It's not a matter of the absolute numbers; it's the ability to make a real guarantee. Of course, the bigger the number, the harder it is to make a guarantee at all, because there are more things going on, and are you sure you got all of them? But with a real time OS, there are far fewer things that happen in such a case, and so it's easier to calculate and to prove that yes, it really will respond within X cycles.