Rupert, what part of the 401 do you drive? I don't mean it as a dig, I'm just curious because the 401 you describe is very different from what I drive. In my experience, the Waterloo area has been the worst for slow drivers, the Hamilton area has been the worst for high speed slalom drivers, and Toronto has a good mix of both. Around the 50 km mark on the 400, people start remembering how to drive again.
And yes, the slow Waterloo drivers are usually old people, with the occasional very young, very beginner driver on their first trip to university away from home, too nervous to keep up with traffic.