Im using xinerama setups since many years and i was under the impression that it is just a fact that xinerama-setups are horribly slow.
Until i saw this:
You can see a 6-screen setup with compiz and it works flawlessly and super-fast.
That guy is using a 3-way sli board and 3 identical nvidias but i still dont understand how compiz is working on this.
I currently use a onboard nvidia 6150 with a pci-e nvidia 7600 and frankly it sucks balls its that slow and annoying to work with.
So i guess i have to upgrade my board and gpus significantly if i want performance like this but i will not even think about it unless i know its gonna work as well as in that video.