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Linux vs Windows tested in 10 games - Linux 17% faster on Average
(video.hardlimit.com)
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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They do work without steam. Steam has just streamlined the process.
And of course you'd need a compatibility layer (e.g. wine/proton). But those aren't dependant on Valve. Even though they've definetly benefited from their involvement.
You can check community reports on http://protondb.com
It's an estimation since proton has made such giant leaps. When a new game releases, you can assume that it runs on linux.
Protondb also has some hard numbers. There are some 12000 games that are playable on the steam deck. Also something like 70%+ of the top 1000 steam games are platinum or gold rated which means they work out of the box with zero or minimal effort.