We noticed an odd behavior on Windows, when was not set (default = 1). The frame rated was much lower than expected. It is always divided by an excact integer value, e.g. if we render with 60Hz:

60/6.67= 9
60/12.00=5

If we set QSurfaceFormat::setSwapInterval to 0, the frame rate is 60fps as on MacOS and Linux with value 1 set.

I assume, that some random integer is set on Windows, when VSync is enabled (SwapInterval != 0).

This was seen using the QOpenGLWindow class in our project.