[OS X] Interacting with the GUI causes a drop in FPS

Interaction with GUI in OSX slows FPS. See the attached example.
It runs on my Mac at 30 fps (no user interaction), but when the user is dragging the slider with mouse, the fps drops to ~15 fps!

Comparision of Qt versions (QWidget used instead of QGLWidget):

Qt 4.8.7 (snapshot)

• no interaction: 58 fps
• mouse interaction with slider: 58 fps

Qt 5.4.1

• 30 fps (this is also strange, why is this two times slower?)
• 15 fps

This issue is Mac specific, because on Windows it runs at full speed.