[kourt]

trend toward more CISC-like hardware, because designers have figured that operations that would've been done in software can be done faster with dedicated hardware.

And since clock speeds stagnated a decade and a half ago, but Moore's Law still allows more transistors to be packed onto a chip, adding dedicated hardware for specialized operations is about all designers can do.

[wtallis]

It's not like we reached any sort of limitation on individual transistor switching speed, though. The problem is that you can't evolve a product line by removing features. You've always got some feature or instruction that is the limiting factor for clock speed; some longest chain of gates that need to settle on the output value before the next cycle. You might be able to shorten that chain by breaking up an operation into several instructions or by reducing the number of registers or addressing modes, but that breaks compatibility. On the other hand, anything that can be added without lengthening the critical path and can be powered down when not in use is fair game.

[sliverstorm]

Individual transistors are getting faster, but wires are not. Wire performance has not been keeping up with the transistors since something like 90nm (2004), hence the "telescopic metal stack"

Clock speeds are in part chosen based on optimal path length (measured in gate count). The longer the path, the less is wasted on flop overhead. The shorter the path, the faster your clock and the more flop overhead. Features or instructions are not important to this equation, because you can break a feature into multiple stages divided by flops. So the fact that frequency has stayed about the same should tell you that path length (gates per cycle) has stayed about the same, which in turn means gates have probably stayed about the same speed for many years!

Next, let's not forget about the power/heat envelope. Modern transistors are mind-numbingly fast if you juice them with 5V, but 1kW CPUs are generally frowned upon these days. We continually squeak down the supply voltage to save power and meet the heat envelope.

Now, you might be thinking about all the extra transistors that are added to newer designs. "Forget about path length," you might say. "If we cut crap features and got rid of those extra transistors, we could afford the power cost of turning up the supply voltage, and get faster!" Alright, now you're thinking smart! But 90% of modern CPU area is spent on graphics and cache, not x87, MMX, and AVX512.

I could go on, but I cannot properly cover all the aspects in one comment...