- What else is running?
- What is the screen resolution and settings? IE what FPS are we talking?
- CPU Temperature when it is at 100%? Clock speed? What CPU cooler?
If the clock speed is low, below base speed, and the temperature is high then you have a cooling problem. If you have a water cooler, check that the pump is running, even if it is running doesn't mean it is pumping properly. Could be a trapped air bubble or it is clogged up with debris.
If it is an air cooler, it may have lost contact with the CPU, remount the cooler with new thermal compound. (Same could apply to a water cooler)
If you are trying to run a very high frame rate with a low screen resolution/settings that takes a lot of CPU. Not really much you can do about that. Not much better than the 7700k aside from the 8700k/9700k, 9900k is a bit in a league of its own due to price.