I've got no problem with the mechanic, but 20% seems like a little bit too much
For our turret building friend. I get what they were kinda going for and while this isn't a perfect comparison, but look fiddlesticks 25 / 30 / 35 / 40% ms, but only after he stops moving for 1.5 seconds, and only for 1.5 seconds at that...
Like i get what you're going for but maybe try 10%... or you know, figure out how much mobility is actually worth so you can properly attribute the appropriate amount of kit budget to it.