I also have this problem with Kubuntu: 23.04 on a Dell Precision 5750
The message in Kubuntu Plasma is "Performance may be reduced because the computer is running too hot"
Update: not sure what fixed it, but I booted into Windows + turned Intel Turbo On in the BIOS for now the speed is higher + the warning disappeared.
I also have this problem with Kubuntu: 23.04 on a Dell Precision 5750
The message in Kubuntu Plasma is "Performance may be reduced because the computer is running too hot"
Update: not sure what fixed it, but I booted into Windows + turned Intel Turbo On in the BIOS for now the speed is higher + the warning disappeared.