yea i love this topic, your question makes me think that you are thinking about the game wrong, ill write this instead of answering your query:
assuming you mean the opponents shield is up:
if youre talking about about short hopping and doing a down air, i wouldn't fastfall it all, use the time instead of hitting down to space yourself to the right or the left and get out of grab range or behind him to mess him up
if youre talking about full jumping you shouldnt be doing aerials on an opponent from full jumps when the opponent is on the ground he can shield grab you pretty easily if youre playing most characters, what you should do is land and grab him out of his shield
assuming the opponent messed up and your aerial is a guranteed hit:
if youre talking about a shorthop aerial i would still just use the time to space instead of trying to fastfall, most hitstun lets you land comfortably without fastfalling
if youre talking about full hop downair being fastfalled thats just timing practice you gotta learn, its pretty delayed
In general you never want to get used to timings instead you should just always watch when your character hits the ground
E.G. hitting both iceclimbers' shields is a different timing than hitting just marths's shield which is a different timing than hitting all three other players with your downair when they are clumped together in a team's match, so just watch your character and hit l when you see him bout to hit the ground
btw ALWAYS delay your aerials when the opponent is in his shield and delay your aerials as much as you can in other situations, this way you wont get shield grabbed or other gay things. Delaying is easier to do when you dont fastfall btw.
there are times when you need to do aerials really fast but those are pretty easy to figure out
