Brian Kelly's been pretty good at identifying diamonds in the rough.
Some of these "slow" guys the likes of Julian Love and Will Fuller simply needed strength and conditioning in order to realize their potential. Some of these guys are still growing, after all.
Also, you can't discount the possibility that their S&C coaches in high school were all but non-existent, or perhaps they weren't taught the right way. Once you get to the college ranks, strength and conditioning becomes much more serious.
To put it this way, Will Fuller became quite a burner even under the suspect teachings of Paul Longo. Julian Love shaved a quarter of a second off his high school 40 yard dash time by the time he left as a junior, with only two years of S&C under Coach Balis. I do believe that with another year of solid S&C, he could have hit that coveted 4.40 mark.
It's pretty much a given that Clarence Lewis can become a better athlete under the teachings of Matt Balis. He's going to be a longer term project, but he reminds me of a 2 star WR we recruited in 2002 who turned out to be a solid, 2 year starting cornerback for ND (Mike Richardson).