With the SteamOS Update 133 comes a kernel upgrade that pushes their Linux kernel to the tip of the steamos_kernel branch. The newer kernel code has new network drivers and also reported Intel graphics performance improvements. On the graphics front, Valve also pulled in the NVIDIA 340.32 proprietary driver that has various fixes over the earlier driver release.
This latest SteamOS Alchemist Beta also features the introduction of VA-API support. As explained a few days ago, Valve added VA-API for in-home game streaming to complement their existing VDPAU video acceleration support. The VA-API support will primarily benefit Intel users and lead to lower CPU usage during video game streaming due to offloading the video encode/decode processes to the GPU.
Last but not least this latest beta also has various package updates primarily to fix security issues and match upstream package updates from the Debian Wheezy base. More details on today's 133 update can be found via the Steam Community discussion group.