The Nouveau driver has long had the GLXVBlank option for the xorg.conf to synchronize GLX clients to VBlanks. The option is motivated to reduce tearing but can be an issue if your GPU isn't fast enough to keep up to the monitor refresh rate (or you're trying to run benchmarks).
With Nouveau trying to be consistent with the other drivers and providing a better user experience, this option is now being enabled by default. Those wishing to disable this syncing to the vblanks / refresh rate can set Option "GLXVBlank" "Off" within their xorg.conf -- the option is treated similarly to the Radeon driver's SwapBuffersWait option.
The change to the xf86-video-nouveau driver happened this morning with this Git commit.
Other important Nouveau news this week includes support for the PRESENT extension, Maxwell support was added with GLAMOR acceleration, and yesterday I shared the first Nouveau re-clocking results with Linux 3.16.