After heavy experimentation and scouring this forum, e.g.:
I have concluded that there is something wrong with the nVidia driver that results in horrible 2D performance when using multiple video cards with multiple screens in a Xinerama configuration.
In my case, I have AGP FX5200 128MB and PCI FX5200 128MB video cards, each with two video outputs, for a total of four video outputs (four "Screens" in one sense of that ambiguous word). They are connected by analog VGA connection to four flat panel displays arranged horizontally in one row. The goal is to use Xinerama for one large desktop that spans the four monitors. (Hence, TwinView is not a factor.)
I have no problem getting the setup to work, except that on all of the screens the 2D performance is, as others have characterized it, "like molasses". For example, if I scroll a simple web page in Mozilla (this is Red Hat 9.0) it takes 1-5 seconds for the web page to render correctly. If I move a largish window around, either within one monitor or across monitors, it takes several seconds to repaint the screen. In both of these cases the CPU usage of the process named 'X' (the X server) jumps up to 99% for extended periods of time. (I have a 3GHz P4 on Intel 875 motherboard.)
I have another system with the same configuration except that the AGP card is a Matrox G450, and the performance on this system, while not amazing, is much better, even tolerable.
So, it looks like there is simply something wrong with the nVidia driver for this configuration. If I disable Xinerama, then suddenly all four screens run like one would expect this hardware to run; unfortunately, I need the functionality of Xinerama.
What do I want from you, valued forum participants? Any stories that confirm my conclusions, or that contradict them; any word from an nVidia representative that this is a known problem or is being addressed; any possible solutions besides replacing the nVidia AGP with the Matrox AGP; in short, am I right? And can anything be done about this?
p.s. I have not yet tried compiling a 2.6 kernel for this system, I am planning to do that soon. Any idea if this is likely to help?