The fan air blow speed is well above 10mph So I think it is good at all speeds. It is a very strong fan. Also, alot of the air actually goes diagonally at the back near the exhausts. That's the heat sink region.
fan also has a ducting benefit at speed. I once stopped after a motorway run in winter and fan did not come on it is so effective.
I have wateched the voltage drop when fan is on. I think it gets gradually stronger depending on the temp.
mine goes on at 14.1 with no lights at idle. But sometimes as low as 13.6.
