This is an issue I’m seeing on both the Retail and Classic client. The log in screen and character selection screen are both locked at 60 fps. I’ve tried messing with the background and foreground frame limits and tried disabling them entirely. In game, the frame rate behaves as expected, but it’s just happening on the log in and character menus.
It’s intentional. The game menu is actually pretty graphically intensive and used to just sit there melting GPUs for no reason and wasting a ton of power. I think they changed it around the time Shadow Lands came out.