Multiple X Screens --> SLOW 2D Performance

[calavera]

I have the following hardware setup:
1 AGP GeForce FX5200 128MB with 2 Monitors connected
1 PCI TNT2 M64 32MB with 1 Monitor connected

Im running Gentoo with XFree 4.3.0 and nvidia-glx 5328.

If I use TwinView the performance is ok but i dont get a proper Xinerama setup. Either the third screen (PCI) is a seperate desktop or the first two are seen as one big screen.
So i wanted to setup up the driver to use the AGP card as two X Screens (as described in the README) and then use XFree Xinerama to put it together.
Which works fine except that performance now is REALLY slow.
I'm not talking glxgears or anything, but simple 2D stuff (like Konqueror or Firebird). I can sometimes literally watch the lines of text appear on screen.
Turning of Font-AA helps a little but performance is still nowhere near where it should be.
XFree uses a lot of CPU and I get the occasional program lockup after which XFree starts really hogging up the CPU.
Disabling or changing the AGP settings did not help either.
The XFree logs do not indicate errors or warnings.
I also tried several driver options (or combinations thereof) - all to no avail.

Thanks in advance

[surfed]

try using the open source nv driver for the pci display (2d) and the nvidia driver for the AGP card (3d) works for me.