Well, Elden Ring had a bug in it that killed performance, Proton was able to fix it without touching the game itself and resulted in Linux performance being markedly better.

Then with Starfield it performs about 30% faster than windows consistently.

I can force AMD FSR on any game (and I have an Nvidia card) to get a significant performance boost with no visually detectable loss in quality.

The list goes on.

It's 12,000 and those are rated as "playable". The majority of games on Steam would be playable out of the box, but Valve is being cautious with their verified program.

ProtonDB has over 18,000 user submissions for playable games.

There are many games in my library that aren't listed as Steam Deck verified or even on ProtonDB and they just work.