The phrase "running in pads" doesn't just mean the physical constraints of wearing the football uniform, it's about running down the field with a man in front of you trying to prevent that from happening. 40 times are somewhat informative, but you are really over emphasizing them here. it would also follow that people who are fast aren't automatically rendered slow by having a defender in front of them, and have a considerable advantage against slower players. there are other factors at work, but speed is important, as you have said. this is why they are timed in the 40. although i wouldn't really call the 40 a great measure for inclusion, as primarily all good receivers can run fast--not all fast guys are good receivers. it's a better method for exclusion, or identifying major flaws.