Don't give up and be patient. When I returned to the game over 15 years ago or so, I was under 250 rpms. And through coaching and proper fitting over the years, I slowly increased it. Then being stuck on 300+/- for years, I wanted to up it up again. It was a rougher experience this time around. I injured myself (wrist) in the process and forced it along the way doing it on my own.
The injury was a blessing in a way, somehow, along with seeing Mark Baker and countless practices with video, I slowly increased my revrate by doing less. I minimized my approach with less unnecessary movements which MB pointed out and JustRico also adjusted my grip. I worked on repeating my approach, being aware of my hand at release, and even added a little "whip appeal" as I let go the ball.
My speed didn't suffer in the process. With increased RPM's, my speed is still 16.5 average down lane. Also, through this process I can control my speed ranging from 15 to 18 mph. With my revrate, I'm still learning to control it with repeatability without being fluffy lol.
BUT LET ME TELL YOU THIS, there can be a down side.......
Because this increased rev rate of mine is fairly new to me, learning this in the last 3 years or so, and getting it down this year........ I have found myself to be trap more often between the sprayers and straighter players. Tweener hell. But when I'm on line, it's a thing of beauty.