I can see how a fullscreen window has quite some impact on rendering times on my Retina display. I don't have the effect that using the secondary display is slower, but I was surprised to see that I get a frame rate of only 16fps on either display. Drawing a fullscreen rectangle takes about 1ms, so that's not the issue. But diving deeper, copying the back buffer to the front buffer (macOS always has builtin double buffering) takes most of the time. 

In the comments I saw that Manolo has added some code that special-handles `Fl_Window::make_current()`, so it is definitely a bad idea to call that anywhere. But even after deleting that, the UI is still at 16fps. Maybe Manolo has an idea? Maybe removing make_current() is a good-enough fix?