How to reduce the lag in OpenGl?

Question

I wrote a little application, which replaces the cursor with a hand drawn cursor. Therefor i used a QOpenGlWidget. For the animation i use the frameSwapped signal:

connect(this, SIGNAL(frameSwapped()), this, SLOT(update()));

Till now i don't use any specific OpenGl function, so i just override the paintevent analog to a classic QWidget.

Qt documentation:

When performing drawing using QPainter only, it is also possible to perform the painting like it is done for ordinary widgets: by reimplementing paintEvent().

void Widget::paintEvent(QPaintEvent* event) {


    // Draw Cursor
    POINT LpPoint;
    GetCursorPos(&LpPoint);
    QPoint CursorPos(LpPoint.x, LpPoint.y);
    CursorPos = mapFromGlobal(CursorPos);
    QPainter Painter(this);
    Painter.drawEllipse(CursorPos, 20, 20);


}

I filmed the result with 240 fps and recognized that my drawn cursor is 2 frames behind the windows cursor. It is not important, that there's no lag at all. But only one frame would be great. And it would be great if i would be able to quantify the lag. Such that i know it's one Frame +- the duration for rendering. I already read this, but i'm not quite familiar with OpenGl. And i don't think i can use this with a QOpenGlWidget. Maybe somebody has an idea how to decrease the lag to only one Frame.

Update 1: I did some research and tried a lot with moderate success. My latest version:

connect(this, SIGNAL(frameSwapped()), this, SLOT(animate()));

void Widget::animate() {
    makeCurrent();
    QOpenGLFunctions* f = QOpenGLContext::currentContext()->functions();
    f->glFinish();
    //std::this_thread::sleep_for(std::chrono::milliseconds(12));
    update();
}

I use glFinish() to sync CPU and GPU. Now the drawn cursor is about one frame behind. Sometimes it is even better than one frame. But there are skipped frames, so the drawn cursor does not move at all. Overall there is no consistency. I still have some trouble understandig exactly how OpenGl updates. Maybe some more information: setIntervall is set to 1 and it is double buffered. I think maybe it is a problem not know what Qt exactly does. Calling update only schedules an update and i don't have controll on how the buffers are swaped. Maybe somebody has more experience on these issues. I can say that the paintevent/paintGL is called every 16 ms. I added std::this_thread::sleep_for(std::chrono::milliseconds(12)); to have less delay between render and the actual mouse position. That helps to reduce the average latency. But for me it is nearly impossible to make a solid prediction on what will happen in the next frame.

Update 2: I posted a similar issue on Qt forum. Even though it's not a direct answer, it contains some helpfull information.