To me that sounds strange, given that the Oculus recommended requirements say i5 4590 and a GTX1090/970, not to speak about the minimums.
All the CPU's mentioned are in the same ballpark regarding base clock frequency, the 6600 and 4590 being the slowest @3.3 GHz. The Turbo speeds vary more, from 3.7 to 5 GHz but the question is how long the highest speed can be maintained as it's temporary. Looking at benchmarks the difference between i5-7600k and i5-9900k isn't too big, by architecture the main difference is the size of the cache. As a conclusion the jump from 6600 to 9900 really was significant, from 7600k to 9900k there may not be that much of a difference, to a 9600k even less so.
You say that you have full on graphics settings. The Environment Map is the most resource intensive feature, try lowering the value.
Another thing is to make your Windows as lean and clean as possible. Did you know that the Store updates can take up to 50% of your CPU cycles?