Right. Once you hit a certain feature set (certain clock speed, core count, hardware shaders, minimum RAM, RAM speed, storage speed and capacity) everything after that becomes much smaller improvements. You won't see doublings of the numbers anymore, instead you'll see much smaller percentage increases that, while important in the long run, aren't going to impact users significantly.
So you initially got some major bumps (0%, 50%, 33%, 50%) then it starts slowing down until the jump from 6S to 7 (about 33% again), then it's just creeping up each year. Core count makes a major jump from 7 to 8. But after the 8 there really is no major improvement in the CPU itself (with respect to these raw numbers) so most users won't see a major difference between them on that front.
(NB: I took these numbers from Wikipedia, I'm assuming I read them correctly as I just threw this together in a couple of minutes. There's probably a mistake or two, and I skipped some versions like 5C, XR, and 11.)
Yes, but I got tired of making the tables :) I've pointed that out in some other comments, that was a massive jump and is probably a major reason the 6S is still supported.
Let's just look at the CPUs:
So you initially got some major bumps (0%, 50%, 33%, 50%) then it starts slowing down until the jump from 6S to 7 (about 33% again), then it's just creeping up each year. Core count makes a major jump from 7 to 8. But after the 8 there really is no major improvement in the CPU itself (with respect to these raw numbers) so most users won't see a major difference between them on that front.(NB: I took these numbers from Wikipedia, I'm assuming I read them correctly as I just threw this together in a couple of minutes. There's probably a mistake or two, and I skipped some versions like 5C, XR, and 11.)