I paired my 3090 with a 12600k a couple of days ago. Had an old 2600k from 2011 that was still going strong but had a hard time with dense sceneries in xp11, p3d and msfs, often going down to 14-16 or even worse occasionally. Now I'm sitting happy with V sync on and 97% stable 30 fps in 4k with pretty much everything maxed in msfs (Flybywire), and pretty high settings in xp 11(FlyJsim stuff) and p3d(fslabs/pmdg).
Have a lot of games on steam and all of them that were giving me trouble before are now smooth, I cap everything at 60 on my pc since it seems to solve some problems with older games.
And yes, I had a couple of months of the 2600k paired with the 3090 which is of course complete overkill :) But I got it secondhand some months ago (because reasons) for a slight discount, didn´t want to pass the opportunity by since I was always going to buy the new Alder lake when it came out so I went for it.
As far as I´m concerned it´s a great purchase, especially for mainly single threaded gaming/simming.
Cheers/
Jon