While every request is processed in real-time, by observation it appears that Waze caches some requested routes or major points. This means if it already knows the best route from B to C, and you ask for a route from A to C, it might just calculate the best route from A to B, once it checks that there isn't a better route bypassing B altogether. This does mean that when there is a Live map update some routes might be less than optimal for up to a day as routes are cached and recalculated.