I did upgrade from 2080ti to 6900xt and CSgo fps went from 240 to 40-90. All other games are fine (HZD, BF5, Cyberpunk etc).
My CPU is Intel 7900x ,PSU 1200w Seasonic Prime Plantinum and monitor Samsung G9.
I did use DDU to remove nvidia drivers and installed newest drivers provided by AMD. 6900xt is on gaming mode and all other settings like low power etc are off.
Did test portal 2 and it was fine aswell, so its not engine problem.