Thanks to the new STIM material, the i5-9600K offers excellent overclocking potential, reaching 5.2 GHz on all cores using a high-end air cooler. The temperatures were a bit over 90C, so a water-cooling solution might be more appropriate, still it is quite impressive how the STIM material helps with the overclocking procedure. The OC-ed processor performed similar to a stock i7-8700.
https://www.notebookcheck.net/Intel-Core-i5-9600K-pushed-to-5-2-GHz-on-air-with-benchmarks.338556.0.html