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.