glReadPixels and Nvidia Dualview

I have created a pbuffer which i render to. The scene is very simple (no lights,no texture) I use glreadpixels to read back the rendered picture.
This works great, but if i use two monitors and set it up as Dualview, the framerate drops to <1 fps.
If i switch to Horizontal span, it works great again.
I’ve tested this on GF4TI4600 and GFFX9950, whith Forceware driver 53.03 and 56.56.

Has anybody else experienced this?