I am going to add one thing. I suspect we should also be looking at 10-yard splits as they will reflect a receiver's suddenness/acceleration, which is as important if not more important than their long speed.
Take Evan Engram. He had an impressive 40 time, but he wasn't that sudden and it took him time to reach top speed. As a result, the defense didn't really struggle to cover him as much as his 40 times would suggest.
Take Evan Engram. He had an impressive 40 time, but he wasn't that sudden and it took him time to reach top speed. As a result, the defense didn't really struggle to cover him as much as his 40 times would suggest.