If you are getting that low of a framerate, it sounds as if you are rending in software mode and not hardware. It sounds like Ubuntu is using the default of "vesa" driver for X. I found an interesting forum thread on ubuntuforums.org which links to several other threads. It seems as though the chipset you are using does not support DRI which means you will be forced to run in software rendering.
SIS video difficulties
Quote from one of the threads within that.
* DRI ("Direct Rendering Infrastructure") is the basis for OpenGL and hardware 3D acceleration. Without DRI, X.org/XFree86 will use software rendering for OpenGL and 3D which is really slow.
* The X driver this page is about has nothing to do with DRI. Instead, a separate driver is needed for this feature. This separate driver is developed by the DRI developers. I do not do any DRI related development, hence asking me questions about DRI is useless (and such questions won't be answered).
* DRI is only supported on the 300 series (300/305, 630, 730). A DRI driver for the SiS 300 series is provided by XFree86 4.1, 4.2(.1), XFree86 4.4 and X.org 6.7.0 and later. XFree86 4.3 does not contain a SiS DRI driver; However, installing the drivers from 4.2(.1) works well.
* Once again: There is no DRI/OpenGL/3D support for the SiS 6326, 5597/5598, 530/620, 315, 550, 650, 651, 740, 330, 661, 741, 760, 761 including all model variations with letters in the model number.
* About XGI: Although there is a binary XGI DRI driver for the Volari Vx chips available from XGI, this DRI driver is not supported in connection with the X driver available here or in X.org/XFree86. Hence there is no DRI/OpenGL/3D support for the XGI Volari V3XT, V5, V8 chips yet, unless you dare to use XGI's "own" X driver which comes with the said binari DRI driver. (The Volari Z7 has no 3D engine, so thinking about DRI is moot.)
Keep us informed as to your results on this.