In the less intense OpenGL games, the Windows 7 graphics driver carried a speed advantage over Ubuntu Linux with the latest Intel graphics driver, but under both operating systems the games were generally playable and the difference was only 10~15% on average. With the more demanding OpenGL workloads, such as with Nexuiz and Lightsmark, the Windows 7 software ended up being multiple times faster than Linux. Besides the Windows 7 Intel driver being outright faster than Linux, the Windows 7 driver is capable of running more games (e.g. id Tech 4 titles like Doom 3 and Enemy Territory: Quake Wars) and other software due to its support of OpenGL 3.0 -- where as the Intel Mesa driver is still bound to OpenGL 2.1 with limited support for OpenGL 3.0 extensions -- plus there are features not implemented in the Linux driver like S3TC texture compression.
This phenomenon though is not limited to just the new Sandy Bridge / Core i5 2500K hardware, but previous generations of Intel hardware and all of the Mesa / Gallium3D drivers. These drivers just are not as fast and well tuned as the Windows graphics drivers, which are the vendors' principal interest. If you run these same tests on the proprietary NVIDIA and ATI/AMD drivers under both Windows 7 and Linux, you find the performance is the same (and in some cases the Linux performance is even better). The performance is similar with the proprietary drivers as they share a common code-base with the highly optimized Windows drivers. But, like the Intel Mesa driver, the performance of the open-source AMD/ATI and NVIDIA (via the Nouveau project) Mesa/Gallium3D drivers are no match to the proprietary drivers. With AMD and NVIDIA, the customer at least has a choice of whether they want to use an open-source but feature-lacking driver or a high-performance proprietary driver that is of similar quality across supported platforms.
At the end of the day, the Intel Sandy Bridge "HD 3000 graphics" as found on the Core i5 2500K is a huge step forward in terms of performance compared to earlier generations of Intel integrated graphics. As said last week, its fast and comparable to other Mesa / Gallium3D drivers with discrete hardware. However, today's tests confirm that Intel's classic Mesa Linux graphics driver remains noticeably behind the Intel Windows graphics driver, even with the brand new Sandy Bridge graphics processor. Over time, we will ideally see Intel's Mesa performance close in on the Windows driver performance, but do not expect to see this in the near-term.