why does CPUs/CPU cores seem to be capped at 3 – 4 Ghz since almost a decade?
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Damn these answers suck. So there are a couple of factors here. Probably the biggest one is the death of [Dennard scaling](https://en.wikipedia.org/wiki/Dennard_scaling). Basically, shrinking transistors no longer provides the same speed/power benefits as they used to, and compounding the issue, the rate at which we see those shrinks has also slowed down.
But beyond that, in the lead up to the 3-4GHz processors we know today, there was a lot of focus on frequency. Specifically in the mid-late 90s through the early 2000s, clock speeds were increasing *faster* than Moore’s law or Dennard scaling would suggest. This was all from design and architectural work focusing on making faster circuits, and splitting the work between more and more (smaller) steps. But eventually, the “low hanging fruit” there started to run out, and coupled with the aforementioned process issues, we got significant stagnation in frequencies. That said, frequency scaling is not completely dead. We’re seeing chips regularly pass 5GHz today, and 6GHz should be doable out of the box within a year or two. Might even hit 7GHz by 2027-28.
There are, of course, other considerations than the ones I mentioned, like poor wire scaling, power density, etc., but they’re not the dominant factors.
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