That's far from universal. It's true there's been a lot of noise about L4DN having a better framerate, but there have been other games that are terrible, and most are pretty much the same..
OpenGL vs DirectX is going to make very little difference one way or another, performance will be pretty similar give or take a few percent, I think the biggest obstacle to gaming on Linux is going to be decent driver support for graphics cards. Hell, ATI haven't got the hang of releasing decent drivers for Windows yet, never mind Linux.
Hopefully it will come, gaming on Linux is much more viable than it was even just a year or two ago, and much more user friendly thanks to things like PlayOnLinux, but playing most new releases on anything other than Windows is a nightmare.