Right after the official launch of the 9th gen CPU lineup, Intel hired professional overclockers to demonstrate the potential of the new Core i9-9900K processors. Using liquid nitrogen, the 8-core CPU was pushed to 6.9 GHz (7.1 GHz unstable), but performance gains do not look that impressive. The CPU can easily be pushed to 5.3 GHz using watercoolers, though.
https://www.notebookcheck.net/Intel-s-Core-i9-9900K-pushed-to-6-9-GHz-on-all-cores-using-LN.337842.0.html