See my prior comment. With less and less cores, this slope tapering or fall-off would not occur until lower and lower wattage levels. This is due to the fact that the physical size of the silicon comes with a minimal power level required just to sustain the sheer number of transistors and this mostly irregardless of clock speed. Generally speaking, that fall-off corner is proportional to the number of cores, meaning that the fall-off corner does occur until a half of the power/wattage for a half of the cores, until a quarter of the power/wattage for a quarter of the cores, etc.
Sorry, but that is incorrect. Ryzen's single-core/single-thread performance is very close to that of a current generation i5 desktop processor. It is at just about a 5% performance deficit in single-core/single-thread for the same clock speed, making it about ever so slightly ahead of prior generation Broadwell desktop processors. (Note: Skylake and Kaby Lake have identical IPC performance.) Mind you, this is just desktop. Ever since Bristol Ridge, AMD processors generally scale much better into mobile. This means Ryzen should either be neck-and-neck and perhaps even ahead in mobile performance.
Fanless, passive Core M/Y-class Core is indeed impressive, and it even comes close to both current Intel Core i5 and Ryzen 5 desktop processors. It's good, but it's not that good–at least, not yet. The Core i7-7Y75 you cite currently sits at a single-core/single-thread score of around 130 for Cinebench R15:
The second graph is a frequency versus voltage chart and does not give any insight into performance but does give us some insight into scaling and power draw. For this chart, we want as little voltage as possible which equates to as flat a line as possible, not a steep line. The first critical point refers to the first point where the frequency versus voltage slope changes to a higher, steeper sloop. This is most definitely not a sweet spot. The second critical point is even worse and is absolutely not what we want. Actually, anywhere below and before these two critical points (from 2000 MHz to roughly 3300 MHz) is the sweet spot.
A softer, flatter slope for the frequency versus voltage is always the desireable in processor design. Why? Processor power/wattage draw is generally an exponential cubic function of the voltage. In other words, with a minuscule increase in voltage comes an enormous increase in power. That is why we always want to keep voltage demands down at a minimum. Hence, the flatter the slope, the better and lower the power draw as we scale the design up and down. This second chart is just another reason why Ryzen is a very mobile-ready processor design.