Ok, boys and girls! I'm going to lay this out to you and if you got a brain cell in your head, you'll pick it up and use it! This is what Delly is doing to Curry! I could not explain it any better so I'm just going to cut and paste for you to easily comprehend!
"There is ONE Fundamental Basketball Skill in coaching basketball defense that will change the game more than any other - NO, it's not give the ball to your best player and let them go - it's the concept of what we call: BALL DENIAL!
The fundamental skill of being able to DENY an offensive player the basketball, is one of the MOST IMPORTANT things to teach a youth player in coaching basketball defense.
An Offensive Player CANNOT score, if they do not have the ball. We EMPHASIZE strongly, the concept of 1-Pass Pressure to all the kids as we teach defensive fundamentals!
We start with a simple goal - DO NOT LET YOUR PLAYER HAVE THE BALL!
This will usually make the kids THINK one step ahead. Most youth basketball players THINK the first step is- "If my player gets the ball, then I will try hard NOT to let them score."
We want them to think ahead of that, and realize
it is better NOT to let the player they are guarding even get the ball. When coaching basketball defense, developing a mentality of DENYING THE BALL is a most important!"
https://www.howtocoachyouthbasketball.com/coaching-basketball-defense.html
....this article points out clearly that you do NOT have to be "quicker" or "faster" than your opponent to accomplish this feat! It's just a matter of desire and some brains to accomplish this! Especially is this the case in the NBA, where you have a 24 second clock working in your favor! You only have to deny your man the ball for 24 seconds.....generally, even less than that! If he's not bringing the ball up the court, you can cut that by about 8 seconds, and now only have to deny him the ball for 16 seconds! Of course, the main thing you have to guard against is an off the ball pick or screen that could free your man up for a back door play! But NBA players in general, don't have the brains or skill to figure that out!