And yep. You can even run a CPU at full clock all the time, meaning you will never observe a clock transition time. Cloud providers seem to do that.
I agree, there are multiple factors at play. But I don't think it's basically an implementation choice. Certainly it looks like it in some cases (S821 on battery, HSW-E and SNB-E). But it doesn't seem to be the case elsewhere. For example, speed shift lowers clock transition time by taking the OS out of the control loop.
I guess they must be trying to softly dis-incentivize really long chains, but not block them outright? It doesn't really make sense to me...
I guess my point is the "clock frequency ramp time" is really due to the interplay of a bunch of different control systems, some in the OS and some not. And when those systems get mixed together, in a somewhat uncontrolled way (which is the case for most PCs), a huge amount of variability is the result and that's what the article did a good job quantifying but IMHO didn't make clear.
But at the time scales in your plots "how quickly CPUs change clock speeds" is basically an implementation choice.
Just my $0.02