He is slow on flim. What are you talking about? Then cue up the tape of CGM doing anything vertical..............ever!!! It is his percise route running, and contested catch ability that gets him open.
Straight line speed is vastly overrated, CGM is sneaky quick and that combined with (yes, you are correct here), precise route running and his ability to win contested catches and there is no reason he can't make the occasional deep catch.
It's not his game, that is true, but it's not strictly because of his 40 time....