system
Given the following post, respond as if you're commenting on Reddit:
human
Why have CPU clock speeds stopped going up?

You'd think 5+GHz CPUs would be everywhere by now.
gpt
tl;dr increasing CPU frequency is kind of like increasing RPMs on a car engine - it's one unit of how much power a car's engine is producing, but it's not so good for comparing multiple types of cars in terms of how fast they can go or how much they can tow.  If one person is driving a Toyota Corolla at 2000 RPM and another is driving a Ford F150 at 2000 RPM, that tells you very little about which is carrying more weight (IPC\*) or going faster (CPU benchmarks) or has more room (cache?\*).

You can push a car's RPMs higher to go faster, but then you produce **more heat** and it's harder on the engine; generally, there'll be a range of RPMs where it's most efficient and is designed to run stably - *frequencies are largely the same sort of thing.*

You can think of CPUs as having basically five metrics that improve performance:

1. Manufacturing tech - how small can pieces of the CPU be built?
2. CPU Frequency - number of times the core clock 'ticks' per second.  3, 4 GHz, etc.
3. CPU Instructions Per Clock (IPC\*) - number of instructions the CPU can execute during a core clock cycle
4. CPU memory\* cache/cache speed - amount and speed of memory that the CPU can use to store instructions/results
5. Design/layout of the chip itself

The implication being:

1. Make the manufacturing tech smaller and everything else constant = lower power requirements, less heat, faster on/off switching of transistors
2. Make the CPU frequency faster = more power requirements, less stability
3. Make the IPC higher = faster (more work done) at the same CPU frequency
4. More memory/faster memory speed = real-world improvements irrespective of CPU frequency
5. Design/layout = real-world improvements in cost, etc

A 4 GHz CPU in 2019 is actually much faster than a 4 GHz CPU in 2009, because the IPC (roughly the amount of work that gets done per Hz) is higher.  The 2019 CPU would also use much less power per calculation than the 2009 CPU, because the manufacturing tech is much smaller.  And it would feel faster because more of the work can be loaded into very fast memory very close to the actual CPU cores (instead of running back-and-forth to RAM).

Basically, they discovered about 10 years ago that they were hitting diminishing returns on increasing CPU core  clock speed.  So they looked to other areas where they could improve efficiency and/or speed: more cores, better cores (higher IPC), more on-chip memory, smaller tech = less power draw, etc.

For a real example: In 2008, a Pentium 4 at 2.8GHz had a per-thread PassMark score of 633; in 2017, an i9-7960X at 2.8 GHz had a per-thread passMark score of 2337.  Same frequency, drastically different amount of work getting done.