Hey, I don't understand this part, can you please explain?
Ok, if you've never played drums before, and you start working on single strokes, you're going to be able to work up to playing single stroke 16th notes at say, 160bpm within a few hours. However, to get up to 180bpm might take a week. Then, to get to 200bpm might take you months. Before you reach 300bpm, you could spend years practicing just single strokes!
That's what I mean by diminishing returns, as you spend more and more time practicing, your progress actually becomes slower and slower. However, it essentially will never be zero progress. You might have to spend five years to squeeze out another 2bpm, but you'll still be progressing!
If you graphed the speed at which you can play against the amount of time you've spent practicing, you'll get something like this:
Now, I'm obviously just kind of making up numbers there, but something very similar to this sort of progression actually occurs! Now, what I was saying at the end there is that if you actually generated a graph like this, you could fit a curve to it, and see what that curve asymptotically approaches. For example, a negative reciprocal curve looks very similar to the curve I drew up there:
If we fit that curve to the one we had above, we be able to find an upper limit. In this case, it would be somewhere around 1200 - 1300 strokes/minute. If our graph is correct, no matter how much time you practice, you would never actually be able to reach that limit, only approach it.
This is intuitively true, we all know that no one is going to be able to play 1,000,000 single strokes in a minute. Even 10,000 is out of reach. But what about 1500? 1300? There is a limit somewhere though! For non-compound motion, I'd say that the limit is somewhere around 1300, actually. Last I heard, the most anyone has ever done is 1247, by Mike Mangini in 2005.
Anyways, hopefully this made sense. For you personally, I don't know if you're at an actual physical plateau, or you're being inefficient in your motion, or what. There are tons of variables! If you want, shoot me a pm and we can sit down on Skype and I'll see if there's anything I can do to help out! I'm certainly not the fastest, but I'm pretty comfortable with 16th note singles on the hands and feet at 200-220 bpm, so maybe I can help!