There is overall problem with WoW after last update.
RX users in some cases got random crashes on DX12 and need to play on DX11, and from what i recall getting similar problems with FPS like you guys.
For me personally 2 of 3 clients don’t even start on DX12 had to change for retail and vanilla to DX11 and most funny thing that Wotlk running on DX12. Blizzard broke smth at some point on last updates.
Link to RX users topic