While maintaining a cautious 25mph or so on a long, exceptionally steep descent (1 mile, 1300ft), I found my brakes beginning to fade. What to do? Braking less hard seemed a slightly risky option, but it would let me catch up with others in my group ahead. Slowing down to a crawl seemed less chancy but also less attractive. I compromised by rolling on at the same prudent speed, using firmer braking as needed. Not far from the bottom a front spoke snapped and the wheel immediately developed a major wobble, which brought me to a halt rather awkwardly.
When rim heating is the main concern, the best speeds are the two extremes of coasting and crawling. So the correct choice between speeding up and slowing down depends on whether you are already going faster or slower than the worst case speed. If you are going faster, it will help to go faster still. If you are going slower, it will help to slow down even more. So: just what is the worst speed to descend at? Maybe half the coasting speed? But the exact answer will depend on how fast the wheel and pads dissipate heat.
Other questions arise: can thermal expansion of an aluminum rim really be enough to cause spoke or nipple breakage? (For a 50deg centigrade increase, I figure the expansion is about a full turn of a spoke wrench.) And is the main breakage risk on steep descents thermal, or mechanical? (I doubt that it’s mechanical, since only very extreme gradients change the magnitude of the forces exerted on the rim by a big factor.)