That's where a striking game is important. Good strikes create opportunities. I'm not going to out-box a boxer, but if I have a good set of strikes, I can control distance somewhat (adding my kicks in, to disrupt his usual entries a bit). And that's what sparring is for. Mind you, I don't feel a driving need to get to aiki techniques. If he's fast but I can get to a clinch, I'll work what I have there, while I try to keep his movement down, take away that power zone.Nage is not moving fast enough to avoid punches from someone who is specialised in hitting fast moving people.
Same deal as I said to Gpseymor. Get someone who is a lot lighter than you to really go for you. And see what sort of time and space you have to play with.
Because not understanding timing is the bulk of what messes people up.