That gem you fell in love with who got drafted in the 4th round or later isn't going to be a stud. In fact, it's very unlikely they ever become fantasy relevant for multiple seasons. And it's still very unlikely they even put up one relevant season.
It's better you come to terms with this fact now, before your rookie draft. You're better off drafting somebody you don't like but was taken earlier.
But do 40 yard dash times really matter? It depends.
But speed isn't everything, sometimes you have to adjust for size and height. Does that make a difference? It depends.
BUT MY FAVORITE SLOW RUNNING BACK HAD A REALLY GOOD 3-CONE AND SHUTTLE!! His agility makes up for his lack of speed. No, not really, it looks like it doesn't matter at all what his agility score is.
For SS, AS, and 40yd times, the population (N = 377, 473, 235 for RB, WR, TE) for each graph is all players since 1999 who have recorded a fantasy statistic. For draft round graphs, the population (N = 966, 1047, 634) is all drafted players (for those drafted) and UDFA is all players who have recorded a fantasy statistic. The population size for these graphs is over 300.