Well I guess in the wilds of Scotland you can go really really fast on open roads but down here that is impossible!
I have been driving as fast as possible for a very long time now and I have never encountered brake fade on normal roads.
Nothing to do with it.
There aren't too many Mk3s that will do 150 MPH, never mind suffer braking problems associated with high speed braking.
Brakes overheat when you use them hard without enough cooling air to control the temperatures...so repeated hard braking from modest speeds will overheat brakes quicker than brakes with 150MPH whistling past them, cooling them.
It's not about how fast you drive...its a function of how hard you use your brakes, and how much time you give them between doing work, to cool down.
I used to overheat brakes on back roads at 50-80...slowing down to 20-30...as you do on twisty country roads.
So, you can then start to take steps to stop the overheating:
Higher temperature rated pads
Higher temperature rated fluid
Grooved/drilled discs
Brake cooling