Ars Technica has put Valve's SteamOS to the test, benchmarking a few games running under the Linux derivative versus the same games running under Windows 10. Though SteamOS was introduced with the promise of performance improvements over Windows, so far this has not proven to be the case, as Ars reports framerates dropping from 21- to 58-percent for some games under SteamOS. It's probably inappropriate to draw final conclusions from these early results, and this portion covers the discrepancies and tries to put them in perspective:

While these are two AAA games ported to Linux by respected publishers, it's possible the developers simply weren't able to extract the best performance from the less familiar OpenGL and Linux environment. We figured that Valve's own games wouldn't have this problem; if anyone could get the maximum performance out of its Linux ports, it should be the company behind SteamOS itself.

Unfortunately, Valve's own Source engine games showed the same performance hit when compared to their Windows versions. Portal, Team Fortress 2, and DOTA 2 all took massive frame rate dips on SteamOS compared to their Windows counterparts; only Left 4 Dead 2 showed comparable performance between the two operating systems (though there's no sign of those SteamOS frame rate improvements Valve cited years ago).