A lot of the efficiency gains in the last few years are from better chip design in the sense that they're improving their on-chip algorithms and improving how to CPU decides to cut power to various components. The easy example is to look at how much more power efficient an ARM-based processor is compared to an equivalent x86-based processor. The fundamental set of processes designed into the chip are based on those instruction set standards (ARM vs x86) and that in and of itself contributes to power efficiency. I believe RISC-V is also supposed to be a more efficient instruction set.

Since the speed of the processor is limited by how far the electrons have to travel, miniaturization is really the key to single-core processor speed. There has still been some recent success in miniaturizing the chip's physical components, but not much. The current generation of CPUs have to deal with errors caused by quantum tunneling, and the smaller you make them, the worse it gets. It's been a while since I've learned about chip design, but I do know that we'll have to make a fundamental chip "construction" change if we want faster single-core speeds. E.G. at one point, power was delivered to the chip components on the same plane as the chip itself, but that was running into density and power (thermal?) limits, so someone invented backside power delivery and chips kept on getting smaller. These days, the smallest features on a chip are maybe 4 dozen atoms wide.

I should also say, there's not the same kind of pressure to get single-core speeds higher and higher like there used to be. These days, pretty much any chip can run fast enough to handle most users' needs without issue. There's only so many operations per second needed to run a web browser.