Speed matters at WR and CB. On certain routes you most definitely are running in straight lines (Go route, deep post, deep slant, et cetera).
The difference between 4.4 and 4.5 equates to ~3 feet over the course of 40 yards.
CB is almost always in a position of disadvantage because they are playing reactionary and trailing the receiver. If you don’t have speed to make up ground you’re toast.
To me the best CBs can overcome any speed disadvantages with technique and proper understanding of the receiver they are up against. And proper coverages by the DC
Same thing for receivers by proper route running or having the knack for finding the soft spots in zone or being under 6ft-white and playing for the Patriots.
Obviously in a vacuum a receiver running a 4.2 vs a guy at 4.4 will have an advantage in a heads up race. Especially in a cover 0 look. Buts it's often not the case where that receiver gets that opportunity, especially if the corner has safety help or can press better than the receiver can get a clean release.
Speed matters but there's so many ways to actually take advantage of it. Out of all the fast 40yard receivers who enter the draft only a handful ever make am impact with their speed.