In my experience, a great portion of competitive multiplayer games work. Although I have to admit that I mostly play games meant to be played among friends rather than against strangers.

If you are not talking about Steam, which comes with Proton out of the box, I’d recommend to give Legendary a try. It’s basically the same thing, but with non-Steam games. And it’s very user-friendly, like Steam.

VM startup time can be skipped by saving state instead of shutting it down every time.

I would say the worst issue using a VM is with programs that need the GPU (e.g. CAD softwares or games), and software with aggressive DRM.