How do I get real fullscreen?

So I haven’t played wow in years with the exception of some different vanilla private servers which has run great on my 2013 15 inch macbook. I also tried Legion for a few hours a couple of years back and was surprised by how well and quiet it ran, even on quite decent settings, on this old mac.

Based on this I was convinced that Classic would run perfectly as well. Everywhere I look people say that toasters can run it on Ultra Mega Deluxe at 240 fps.

When I tried Classic out for the first time during the stress test last weekend I was surprised how bad it was running, and how my fan started working even at the character selection menu. I went through all the video options and changed everything I could back and forth but it felt like I got nowhere near the performance of the private servers or how I recall Legion ran.

After a while I noticed that I was running Windowed fullscreen, something that I remember would reduce the performance quite a bit on private servers as well, and tried to change it. I guess I this doesn’t come as news flash to most of you, but apparently you can’t change that anymore.

Now it seems like I have to lower my settings quite a bit, and still have a fan screaming at me, compared to smooth and quiet ride at the private servers.

Does anyone know if it is possible to get real fullscreen? Or at least a performance comparable with it?

Tldr: Windowed fullscreen sucks, how do I get real fullscreen or at least the performance of real fullscreen?