Here is the situation:
I have 4 SFML windows, which are inside a container which I have built. The container calls independent redraw methods for each window, starting with the first and ending with the last.
If each window's drawing code contains the lines drawMyCube() OR glClear(...), then the frame rate becomes slow.
drawMyCube() just draws a cube which rotates depending on the value of an sf::Clock object.
If one window calls (either of) these functions, the frame rate is ~60fps. If two windows call (either of) these functions, the frame rate is ~30fps. If three windows call (either of) these functions, the frame rate is ~20fps. Finally, if all four call (either of) these functions, the frame rate is ~15fps.
This looks like a pattern emerging, so I tried removing the functions from 3 of the windows, and calling them 10 times from one window. I was expecting the frame rate to be ~6fps, but it remained at 60.
Does anyone know why this is happening? There doesn't seem to be any effect if I remove any other functions from the window drawing methods, for example, gluLookAt() doesn't seem to slow it down.
EDIT: Frame rate limit is set to zero and vsync is false.
This sounds exactly like vertical sync. Each of your windows is waiting for vertical refresh, which is why your rate keeps getting cut in half.
I know you said that vsync is off, but it's possible that your video driver is forcing it. Check your driver settings.