https://www.techpowerup.com/259688/windows-10-2h19-update-to-have-favored-core-awareness-increase-single-threaded-performance

Intel® Turbo Boost Max Technology 3.0 (which is not available for 9900K/Coffe Lake):

https://www.intel.com/content/www/us/en/architecture-and-technology/turbo-boost/turbo-boost-max-technology.html

Single core clock makes basically no sense for 9900k. Only if you limit the number of threads in control panel for that application. So if you have 2 cores clocked at 5.2 GHz and limit the number of threads to 2/4 in the control panel it should utilize 5.2 GHz.

If the rest of your cores are clocked at 4.8 GHz and you give it another thread, all of those threads will be downclocked to 4.8 GHz and your 5.2 GHz will go completely unused.