Getting open in the NFL is so much more about route running and understanding the defense you're going against than it is athleticism. Jason Witten is not as fast as Dez Bryant, but he gets open a lot more. Also, Jerry Rice was the slow guy coming in to the NFL. It sure didn't stop him.
Please keep in mind I'm not saying a 5th round pick is going to be anything like those guys, I'm just saying, speed alone doesn't get you open in the NFL, other than once or twice a game if you're fast enought to just straight outrun everybody on a fly pattern.