My MPB has had this problem for years every time the ambient temperature gets a bit warmer. It starts in the spring and gets ridiculous during the summer.
The MBP clocks down its processor to 0.8 GHz, making it pretty much unusable. This happens as soon as some process-demanding apps (like NodeJS or Unity) are running on the machine.
However, the CPU temperature is just fine, between 60 and 80 degrees Celsius. It doesn't look like a thermal problem. Thus, I don't understand why this happens?
I tried every hint I found on the internet, including resetting SMC and NRAM, I even re-installed macOS.
Because this has happened from day one, I have been at the Apple Store many times, and the MBP now has its 4th logic board (!) Still, the problem remains.
What else might cause this strange phenomenon? I'm at a loss!