Hi there.
I know many topics are open about it; however, this issue is still now fixed - and it’s just unacceptable.
For the description: the game’s main menu’s performance is poor, while the game itself runs perfectly. Different graphics options do not fix it.
My setup: 2020_02_07_dxdiag - Pastebin.com
What helps: to disable the integrated graphics card via device manager → to launch Warcraft Reforged → to enable it again (because without it, my first display works bad). In this way, the game menu’s FPS is ok. However, this is not a proper solution.
Please, pay attention to this problem and fix it.
No one will fix it, the game is abandoned.
Try to play the game in a 4:3 resolution. This fixed the problem for me.
I had the same problem with my asus notebook.
i7-8750H + Gtx 1060 6GB + 16GB memory.
The game is full buggy, the game menu can’t handle the integrated graphics and the dedicated one… i mean you need to play on desktop computer 
I dont have any issu with weaker specs desktop rig… Have fun…
i use integrated graphics as a passthrough to discrete graphics (connected monitor to integrated graphics out) and maybe it is related to this issue. if the game uses igpu to render menu which causes lag.
This is even worse on macOS. I tried the game on a late 2019 MacBook Pro 15" (i9 CPU + Radeon 560X 4GB) and a mid-2020 MacBook Pro 20" (i7 cpu + Radeon 5500). The game is slow, the menu is slow. What a shame…