It's Not the R2 button.
This has been happening and I'm fairly certain it's on purpose. Why else would your player stop sprinting full speed, and look up, then put his head down and start sprinting again without being tired? This is another form of the "Auto Hesitation" from older 2ks. First noticed it when I had a player with 99 speed acceleration and speed with ball come off the bench, get into a fast break, and be nearly the last one down the court. It's so you don't abuse the speed of players like Westbrook. It's so the (I play on HOF so I'll say the HOF CPU) CPU can catch up and guard you. I think it came in on the same patch the dribbling nerfs came in, what was that patch 6 I think? Haven't found a solution, Used to be do a behind the back or a cross over but they slow you down after that too because 'speedboosting'.