Dell XPS 13 9333 Developer Edition CPU stuck at low frequency

One day my good old Dell XPS 13 9333 Developer Edition (almost 7 years old, now on Ubuntu 20.04) suddenly became sluggish and startup slow. CPU appears to be stuck at 800 MHz, although it used to work at 3000 MHz.

* This does not look like a power adapter issue. I had adapter warnings before, with power light turning orange and a warning at the beginning of startup saying that adapter cannot be recognized and CPU will throttled down. Now the adapter is new, the power light is white, and there is no warning.

* Diagnostics (F12) gives no problems except for telling me that the battery is missing. The battery had died and I physically disconnected it a few years ago. The fan and DC jack have been replaced. But the computer has been working fine until now.

* I assume the problem has nothing to do with upgrades to Ubuntu, because live USB now also starts with low CPU frequency.

* I did a BIOS reset to the factory settings, but no effect.

* Tried "disconnect everything and hold power button for 30 sec", but it didn't help either.

Please help me get my CPU frequency back!