20 FPS could be some vsync multiple. If you can turn off vsync give it a try to see if FPS starts to go beyond 20.
It may also be the new expansion is more graphically demanding so the GPU fillrate is now struggling to drive retina grade displays at native resolution. If this is the case lowering resolution should be able to help frame rate.
It might be a good idea to ask in the World of Warcraft technical support forum. This forum is for Warcraft III Reforged.