9900k at 5.5GHz is amazing! What Vcore and cooling are you using??
I don't recall the vCore off-hand. Not as high as you'd expect. The cooler is the important part -- an external Koolance cooier which is probably itself about a third the size of the full case of the main PC. It has to be switched on 10-20 minutes before the PC else it that often BSOD on booting into Windows, if not soon after.
But as Chris said, it isn't only the CPU. The cache at 5.0 has a lot to do with it, as does the faster RAM. And I've tried to optimise the spread of the 8 core usages across the add-on processes I have to run to support my cockpit system.
Even so, Core 0 is still hitting 100% most of the time -- but that's probably because i run with about 3 times the number of AI as before -- over 200 and up to 300 in busy areas, up from my previous limit of 80.
I plan to limit it again, to 200 or even only 150 so I can ramp up some other things. i'd really like to use Orbs True earth series, but no way that will work well even with this beast.
It's having the three scenery views for ny 210 FOV screen which is the real killer, and the reason for having this monster built! ;-)