Comment 10 for bug 774008

VGA compatible controller [0300]: Silicon Integrated Systems [SiS] 661/741/760 PCI/AGP or 662/761Gx PCIE VGA Display Adapter [1039:6330] (prog-if 00 [VGA controller])

OpenGL renderer string: Software Rasterizer

I suspect these pieces of information point to the problem. The Linux SiS driver has not received much maintenance for some time, let alone development. In fact the 3D portion was recently dropped entirely by upstream Mesa - see http://www.phoronix.com/scan.php?page=news_item&px=OTg0Mg
The first release to go without these drivers will be Mesa 7.12 when released in January. Furthermore, according to http://dri.freedesktop.org/wiki/SiS "There is no DRI driver for "[SiS] 661/741/760 PCI/AGP" chipset..."

"Software Rasterizer" indicates that your system is not using the SiS graphics chip for hardware-accelerated OpenGL rendering (not surprising given that there is no driver), but instead has fallen back to the slower software-based (CPU) rendering.

In conclusion, unfortunately I doubt that there is much that can be done about this. The only slight hope is that future releases of Ubuntu (by then you would probably need to use the lighter Xubuntu or Lubuntu on that machine) will probably switch from using the classic softpipe software rasterizer to the newer and faster LLVMpipe software rasterizer. This has already occurred on Fedora 15 if you want to give that a try - I'm not sure if they have a version with a "light" desktop.