Nouveau Maxwell: Mesa 17.0 + Linux 4.10 vs. NVIDIA's Linux Driver
With the lighter ET: Legacy game, Nouveau is running at over 80% the speed of the proprietary driver with the GeForce GTX 750 Ti. For these older games, the GeForce GTX 750 series does deliver very playable frame-rates on the open-source driver.
OpenArena was just around half the speed for the GTX 750 series on Nouveau and much less with using the GTX 980 at the boot clock frequencies.
With Portal being an older game, its Nouveau performance is much better relative to the proprietary NVIDIA 375 Linux driver, at least for the re-clocked GPUs...