For me, it’s Shared GPU memory.
The one who probably provided me the most trouble and headache over the years is my favourite anno, 1404 history edition
I tried it running with portproton with pirated version works smooth no tinkering.Running on arch linux with hybrid graphics on nvidia mx940.
I usually check the proton DB website, to read the comments and see what people do to fix games and software.
Just recently I used it to get old CAD software to run, had to lower the proton version to 6 or something and it worked.