"12nm" is just 14nm+. GlobalFoundries called it "12nm" because it sounds better.
I also said months ago that if GlobalFoundries promise 10% performance uplift, expect half of that.
12nm is an optical shrink with a slightly smaller Minimum Metal Pitch of 56nm. btw the clocks you hit also depend on physical design. My guess is AMD has done the minimum by directly porting the Zen design to 12nm by going with 9T libraries. AMD seems to have avoided costly redesign by going with higher track height 10.5T libraries as it would probably require more validation and resources. AMD still are limited on resources and have chosen to focus on 7nm Zen 2.