glBindFramebufferEXT slow down framerate


As soon as I bind a FBO (even if I don’t use it, unbinding it just one line after), the framerate slow down from 125FPS (no FBO used) to 75FPS (one FBO bound/unbound).
Worse, if I use one FBO for the whole rendering (instead of rendering to screen), I get 60FPS.

What’s going on ?

I’m using a GeForce GTX260, driver 197.13, Win 7 x64.