I find it a bit difficult to play down-up (with the appropriate "sweeps, etc.), rest-stroke style, when the tempo is slow or medium-slow. No problem picking faster -- my wrist feels relaxed, the pick bounces off the "rest" strings, etc.
When slowed down, gravity is still the same, the string tension is still the same, but the tempo is slower -- seems it's sort of like expecting a horse to gallup slowly.
It's obvious that many of the best players play all down strokes until the tempo gets pretty fast.
In Stochelo's video lessons, there is even a disclaimer that, when he slows down his licks, his right-hand pick directions are not the same as when he plays the licks at tempo!
Is it just tradition? Or for a better sound? Or is it because it's actually more difficult to slow down the bouncy rebound of the strings, etc? What would happen if a horse tried to gallup slowly?
I'm wondering if it's not worth trying to perfect a slow "down-up" rest stroke, and if it would be better to perfect faster down-down playing?