SIAP, but this is a good tactic he describes at about 1:50.
https://www.golfchannel.com/news/brooks-koepka-no-one-has-balls-penalize-slow-play
I actually had to do this with one of my friends a few years ago. I constantly found myself speeding up to compensate for his slow play. One day, I just shut it down and played painfully slow. I think it was the 1st time he ever felt pressure from the group behind us, and he actually began to pick up the pace. That worked for a couple of rounds, but he gradually went back to his slow play. I finally quit playing with him altogether.
https://www.golfchannel.com/news/brooks-koepka-no-one-has-balls-penalize-slow-play
I actually had to do this with one of my friends a few years ago. I constantly found myself speeding up to compensate for his slow play. One day, I just shut it down and played painfully slow. I think it was the 1st time he ever felt pressure from the group behind us, and he actually began to pick up the pace. That worked for a couple of rounds, but he gradually went back to his slow play. I finally quit playing with him altogether.