Downhill braking, what should the balance be?

[Tigerbiten]

Whether descending fast or slow there is the same amount of potential energy at the top of the hill. A fast descent will lose more kinetic energy to wind resistance than a slower descent, so leaving less energy for the brakes to deal with. (assuming both descenders are freewheeling.) Hot components would also cool more rapidly when exposed to a faster air flow.

Whether this makes enough of a difference to matter I have no idea.

Kinetic energy is 1/2*M*V^2.
So if you let the speed get twice as fast before you start braking then you have four times the energy to dissipate as heat in a shorter time/distance after you start.

Short steep hills are OK, you can just let the bike go.
It's the long downhills where you can be on your brakes for 10 minutes or more that you need to be more careful on as you balance speed down vs heat in the brakes.
Depending on the road (steepness, bends, tarmac, length, etc) there is a maximum slope I will let it run on. As it gets steeper I start descending it slower. A 25% down I'm much more likely to descend at 10-15 mph as opposed to 40-50 mph if I don't know the road so I don't overheat the brakes. I've overheated and burnt out two sets of disk brakes in the past from coming down to fast.
Starting at around 2% down and 20 mph, if I double the steepness then I add around 10 mph to speed.
So the best hills down are only around 8% max because I'll top out at around 40 mph, any steeper and I'll have to start using more brakes more often to keep things safe.