I shy away from using terms like "football speed" and "making up for his 40" because those are faux terms to me. There's no real difference between football speed and speed, IMO. Using the term "football speed" is like tossing your hands in the air in frustration and saying you can't explain why the guy does what he does.
The reality is that his short stride and hips make him good with fakes and probably about as fast as most WRs in the 10 yard area...but then where the other guys toss on the afterburners he doesn't have any to toss on.
You can't make up for having a slow 40 time. You just do what you're good at doing. Anquan Boldin, well first off when he's healthy he doesn't run the same 40 as he did before the draft, and second off he does what he does best, break tackles and play with a great 2nd gear. He doesn't make up for his slow 40 time, he still can't consistently separate deep or anything, he just works around it.