Thanks for the note, Chris. I'm guessing that one or more of these factors were at play in our ride down the Cabot trail:
1. Rain (with very heavy fog). Rim brakes were not working as well as they would have in dry weather. So more work needed to be done by the drum brake.
2. Arai brake was shaved.
3. When the brake got very hot (the outer (shaved) rim of the drum brake was way too hot to touch--but, of course, I did it anyway), we were concerned what would happen if we kept using it. So we waited on the side of the road for 3-5 minutes for it to cool down. But the pattern kept repeating. We probably stopped 3 or 4 times going down MacKenzie.
Now, as for what we were doing "wrong", I can't think of any explanation except 1 and 2 above. Also, with the heavy fog (we could see clearly only about 10 m before the fog took over--our real concern was that we would be run over by a car), there were no clear sight lines. Actually, the next day, coming down North Mountain (we traveled clock-wise), we had the same problem in good weather. For North Mountain (yes, really a long hill...) there is long relatively straight descent followed by a sharp left turn. That was tough for us as well. Here, back in the Finger Lakes (some steep hills), with the same bike but an unshaved Arai brake, we are fine.
I don't know what the grade was on MacKenzie, but were told some stretches were 15%-18% (no idea if that is correct!). In the Cyclist's Guides to Canada's Ocean Playground, Gary Conrod's says that a hill past North Mountain drops 365m in 3.2 km, but that North Mountain is the fastest descent on the Cabot trail (no elevation changes given)
I can't speak for the Swiss Alps directly, but if they are not a lot tougher than the Cabot Trail I will stop watching the Tour! We hope to visit your beautiful country someday.