If you run anything sub-4.5 then only your playstyle dictates whether or not you’re an NFL caliber deep threat. Guys like Desean, like Tyreek often have breakaway TDs where they’re 5-10 yards behind the coverage. If they ran a 4.4 that’s still an open touchdown. It’s their route running, their understanding of the coverage, their downfield experience, and their intuition/intangibles that make them great deep threats. The speed helps make them slightly better deep threats, but the difference between 4.3 and 4.4 isn’t multiple yards of separation, even in a long route.
Deandre Hopkins at 4.62 is a better deep receiver than 90% of the sub-4.5 guys. Jeremy Maclin at 4.4 was significantly better downfield than John Ross at 4.22. Devonta Smith ran a 4.54, Mecole Hardman a 4.33… but you’d think those were flipped based on how they perform.
The 40 is just to let you know which guys are so slow you consider dropping them off your board. A 4.21 vs a 4.35 will have 0 relevance in how their careers go. Theyre both fast, take the guy with the better tape.
TL;DR: Xavier did not suddenly become a better football player in the last 2 days, but some teams will treat him as such