First off, please understand that I'm one who finds "big game speed" to be incredibly overrated. I've seen a few "speed demons" at WR that didn't amount to a hill of beans in the pros. I find it to be equally more overrated when it happens at the Combine. Anyone can run fast in their shorts and a t-shirt with noone defending them, how good are they gonna be with an additional 10-15 pounds worth of equipment and a 265 pound linebacker or a shutdown, hard hitting safety/cornerback in his face. There's a reason why James Jett never amounted to much in the NFL, while Anquan Boldin, who had one of the worst 40 times ever recorded for a WR at the Combine, has been a perennial Pro Bowler and one of the NFL's best WRs.
That being said, I've said all along Calvin Johnson is the real deal. I think he's going to make someone, whether it be Oakland, Tampa Bay, Cleveland (I still think there's a chance they pick a WR inspite of having Braylon Edwards, even though I think they go QB/HB first) a very happy team. This 40 time just solidified what we've known all along, that he's going to be one of the top players off the board.