The Tech Report writes Windows 8 will offer hardware-accelerated graphics tweaks built into the OS to improve performance:
Graphics performance is a priority for Win8, and Microsoft created new test metrics to ensure a smooth experience for end users. In addition to targeting a 60 FPS frame rate for OS-related visuals, Microsoft started counting glitches, which it defines as frames that take longer than 16.7 milliseconds to render. Looks like someone's been paying attention to our Inside the Second game benchmarking methods.
Microsoft's graphics optimizations focus more on OS elements and applications than on 3D games. Time has been spent tweaking DirectWrite to speed text rendering, and tessellation performance has been improved for simple geometry like rectangles and ellipses. JPEG and PNG images should decode much faster on the new OS, as well. You can see some of the benefits on display in the video embedded below.