So what's the solution? Now it's 2021, is there a solution?
I also have the same problem, my laptop is Lenovo Idea pad Slim 3 (Ryzen 5 4500u), stuck at 0.38Ghz.
I've contacted Lenovo premium care, I was asked to update windows, and I've also updated the BIOS but it's still the same.
Even worse, even when charged or not, the CPU speed remains 0.38 GHz. The laptop must be left (unused) for about 6 hours before it returns to normal, and even then it only lasts 4 hours, after that it gets stuck again at 0.38Ghz.
I tried Throttle Stop software but it says "CPU does not support", finally Throttle Stop can't be used.
I am very frustrated.
Oddly enough I'm seeing this exact same issue with a brand new HP 17z-ca300 CTO laptop - Ryzen 7 4700U CPU. The laptop is always plugged in, running off wall power (not battery). After I shutdown (usually overnight) and boot back up the next morning, the CPU is throttled at 1.6Ghz. Rebooting Windows 10 always fixes the issue. CPU stays at or below 1.6Ghz until I reboot. After reboot, the CPU boosts all the way up to 4Ghz per specs. Definitely a weird problem.
I have 2 other AMD Ryzen laptops (Ryzen 5 3500U and a Ryzen 3 3200U laptops - never encountered throttling on either) and have only ever experienced this on this particular HP laptop. I suspect firmware as well at this point. I've reported the issue to HP, but who knows how long it'll be before a firmware update. At least the problem is easily fixed by a single reboot.
Im also experiencing this issue on my HP pavilion 15-cw1500sa with a ryzen 3 3300U
there is a newer UEFI version available than what im running but i cant update to it as HP's tool isnt working to make a update USB (im on linux so i have to use another machine to make an update USB stick) so i cant test it.
Ive noticed it has happened a lot recently and also happens most of the time when i connect my power supply to my laptop when its running, it also sometimes happens when i boot up with the power supply connected.
I know for a fact im not thermal throttling and from what i can tell, this issue is very random. Having this issue is highly annoying, especially when im trying to get my work done at college
Here's what fixed my issue...
Disabling Windows 10 fast startup - that explains why the problem would surface after a shutdown, but a RESTART would fix it. Fast startup was the culprit.
Control panel > power options > change what the power buttons do > uncheck fast startup (see screen shot below)
Also make sure your AMD chipset drivers are current, and that you're using one of the AMD Ryzen power plans in power options (if available).