When we are doing overclocking it is possible that the temperature rises to 80 or even 90°C despite the use of the most advanced water cooling system. Beyond that, the temperature is said to be dangerous for the processor. And to increase its lifespan, it is advised to return to lower frequencies.
With such temperatures, is the performance of the processor tainted?
Yes, but at infinitesimal proportions, ie of the order of a few micro percent. The temperature slightly interferes with the voltage.
So no, performance does not change, unless the temperature is such that the processor is about to create errors and restart the machine or bring up a blue screen.
To check, make a test on the benchmarks with a fan cooler, then a water cooling (which cools a little more).
It is advised to stay below 70-80°C max to maximize the life expectancy of the processor.