SwapBuffers & fps

I'm writing a program in which more than one window can be opened. I call the following function every frame :

Code:

---

void swap_buffers(void){
        Window *win = G->window->first;

        while (win){
                wglMakeCurrent(win->hdc, win->hrc);
                SwapBuffers(win->hdc);
                win = win->next;
        }
}

---

Here, Window is a self defined structure which is linked in a linked list, and contains the handles to the rendering context and the device context.

The problem is the following :
- when I have 1 window opened, my framerate is 85 fps (on average).
- when I have 2 windows opened, my framerate is 42.5 fps.
- when I have 3 windows opened, my framerate is 28.3 fps.
- when I have 4 windows opened, my framerate is 21.25 fps.
=> fps = my monitor refresh rate devided by the number of windows opened.


I suspect that the problem lies with the vertical sync which limits the pfs to (in my case) 85 fps. I don't mind having a maximum of 85 fps, but when I open 8 windows, my fps is about 10 fps which is to low. this is probably caused by calling SwapBuffers(...) more than once each frame (once for each window). But I don't know how I can solve this, because all windows have to be redrawn each frame.

Can anyone help me here ?

Thanks in advance.