I think a lot of laptops do this because their cooling system is too weak for the CPU.

The Lenovo Thinkpad t480s comes to my mind: with factory settings, the CPU was unable to reach its advertised boost frequency nor maintain its max non-boost frequency for more than a few minutes. After 5 to 10 minutes, the machine was usually running at 300/400mhz under its advertised clock speed.

Undervolting helped a lot but could not fix the issue completely, under clocking was the only solution to maintain a consistent clock speed.

IMO, at least with x86, there is a point where it simply doesn’t make sense to cram higher clocks into a laptop. The last Intel MBP could be had with an i9, and IME if you left turbo enabled, even at idle the fans were annoyingly loud. If you pushed it, you’d get a quiet jet engine as they struggled to keep temps just below 95 C.

It was usually less annoying for me to just disable turbo, and only turn it on if I really needed the boost.

Thinkpads, for a long time have had a fan speed that's possible, but never reached by the firmware's automatic mode. On Linux, this sets the fan to its true maximum:

    # echo disengaged > /proc/acpi/ibm/fan

I use the thinkfan utility to run the fan more aggressively, including 'disengaged' over 70C.

How's your hearing? If any left.

I put in foam-tipped earbuds if I'm doing a big compile or something, but the Ryzen 6850u runs pretty cool most of the time.

Indeed, and for the t480s we could undervolt the CPU to help with thermals but these are not normally accessible to the end user.

So if you are an average user of a Lenovo laptop without technical skills, you probably have a machine that does not meet the advertised performance, and no way to fix it.

This is likely the case with all but gaming laptops. Most users won't care about performance under consistent, heavy workload, as much as it loading that .docx in 2 instead of 6 seconds. That burst of performance is what makes the system feel snappy and fast. If the user is happy, it was well worth the money too.

plenty. But normally the degradation is more gradual and automatic. For example, as the innards of your clothes dryer gets clogged up with lint, at some point the thermal cutout will cut in and out to briefly reduce the heat.

End result is your old clothes dryer starts taking longer and longer to dry clothes.

A lot of laptops do this when running on battery power. Do they pop up notices telling users they are running in reduced power mode?

I recall mine sometimes doing so if I unplug while it is on. Never occurs while it is booted on battery.

Laptops and phones also notoriously slow down to manage heat, and that is also never told to the user.

Clog the heatsink on just about any PC and it'll happily and silently thermal throttle itself.

That stretches the definition of silent. On MacOS you need command line utilities or third party software to observe throttling, but on Windows this information is easily viewable in the performance tab of the task manager, and Linux exposes this information in /proc.

Being buried away in technical debugging tools is 'silent' for 99.9% of users.

A common task that repair shops do to "fix a slow PC" is clean dust from the fans and heatsinks to reduce throttling. Users don't know or understand that this is happening, they just know their system is slow.

Maybe this is just me but I always assumed USB cables had indefinite lifespan until I happened to read a datasheet.