Updating message in status bar or scrolling list widget slows down repainting when QOpenGLWidget is used. Please try the test software (V-Sync must be turned on). Updating QLabel, QSlider and QOpenGLWidget simultaneously doesn't affect the speed, but changing message on status bar or scrolling QListWidget slows down rendering. It looks like repaint is not scheduled in message queue. Updating label in the status bar (permanent widget) is also fast.
Since https://codereview.qt-project.org/#/c/140945/ on XCB moving the window locks the application which uses QOpenGLWidget. The expose event is sent to the application by every window position change.
I noticed that using QOpenGLWidget results in higher CPU usage (entire window uses OpenGL), is it correct? I thought that OpenGL should speed up rendering.