Valve has been hard at work getting Steam Machines out the door and onto the market running on the Steam OS. Powered by Linux, Valve's machines and OS have been an attempt for the mega-popular developer/distributor to get gain more ground as a dominant format in gaming. However, in a recent test Windows was able to outperform SteamOS in nearly every way.
Conducted by, the test revolved around a dual boot machine equipped with Steam OS 2.0 and Windows 10 64-bit OS. The operating systems were run on seperate drives, but used the exact same hardware for both tests to keep the environment static. The results came out in Windows' favor just slightly on some standard stats that probably wouldn't make anyone bat too much of an eye, but when it came to game performance, the difference became drastic.
Both systems were tested on Middle-Earth: Shadow of Mordor and Metro: Last Night Redux because of their availability on both systems and their generally high graphical requirements. Windows 10 outperformed SteamOS 34.5 FPS to 14.5 FPS on the highest settings in Shadow of Mordor and again outperformed 9.5 FPS to SteamOS's 4.2 FPS on maximum settings in Last Night Redux.
Even further, the test took many of Valve's own Source-powered products and put them to the test in 2560x1600 resolution and ran tests at the maximum settings for each game. Left 4 Dead 2 was the only game that came close to Windows 10 with SteamOS just one frame under. Team Fortress 2 and Dota 2 gave Windows 10 at least a 10 FPS edge and Portal provided a massive difference of 107.1 FPS for Steam OS to Windows 10's 146.2
Steam Machines and SteamOS are still a relatively young base for games compared to Windows which has been in the game for a long time. Valve will probably find exactly what it needs to optimize SteamOS to its fullest, but it still seems to have a ways to go in catching up.