This will change at some point
If you game in Linux using Nvidia cards the performance is similar or better than in Windows, this is because the proprietary Nvidia driver + OpenGL 4.5 implementation is really good, closed source but good. Also it depends if the game runs as native OpenGL or through a translation layer from DX9/10/11 to OpenGL. Many AAA games run through some such translation layers with poor results.
If you game using ATI, the proprietary drivers for gaming are not as good as in Windows and games will be slightly slower than in Windows.
The Open source driver for ATI is excellent for Desktop usage, for gaming it depends on the game and what OpenGL requires, (some commercial games will not work without OpenGL 4.1 at least) Mesa sits for ATI cards at OGL 4.1 depending on which distribution you run, the majority of Distros currently run OpenGL 3.3 (If you want to compare it with DirectX it would be comparable to DX 9)
Things are less than ideal, but if they improve during this year as much as they have during 2015 things are going to get interesting indeed.
I mostly game on Linux now, I haven't been a heavy gamer for a long time, last year I bought of Steam all I could get for Linux (Borderlands, Witcher 2, Metro, Bioshock Infinite) I have played most of them at 45/60 fps using Nvidia cards in 1080p and they ran way better than on Windows on the same machine, I was so pleased I removed the last Windows install I had on my Living room's computer.
I do not care if the games are old though, but that its me, I understand other people have other expectations, aspirations when it comes to AAA gaming.
I had a helluva ton of fun playing "Not a hero", "Broforce", and "Battleblock Theather" this Christmas (fun 2D platform sidescrollers) on Steam. I do not miss Windows, from now on if a game port does exist on Linux I will buy it, if there is no Linux version I will not play it. Once again that is me.
I get what Valve is trying to do with SteamOS, they are trying to create a console where HW vendors can not charge gaming companies a billion for the rights/tools to develop games, they are trying to create a new market with a level playing field from scratch, it may or may not work, but they are trying, and certainly will take time to get there if it works, I do not see why they could not succeed if they persevere.
On the technical front Linux has issues with its graphics stack, but the problems are perfectly solvable, and many are being addressed, the improvements on most drivers, + Mesa (FOSS OpenGL) have been sort of amazing on 2015, compared to the poor state of what we had before.