How to update gui and render to opengl simultaneously without stutter

[wakiki]

In my app i get information from different worker threads via signal-slot and write them in my widgets every 500ms:

m_timer->setInterval(500);
QObject::connect(m_timer, &QTimer::timeout, this, &SystemManager::update);

void SystemManager::update()
{
    m_tabWidget->process(); //update gui such as QLineSeries::replace or QTableWidgetItem::setText
}

In an additional window i have a QOpenGLWindow running (i also tried with QOpenGlWidget with same results), which renders animated objects in ~120fps.

GlWindow::GlWindow(QWidget *parent) : QOpenGLWindow()
{
    QCoreApplication::setAttribute(Qt::AA_UseDesktopOpenGL);
    QCoreApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
    QSurfaceFormat format;// = QSurfaceFormat::defaultFormat();
    format.setSwapInterval(0); //vsync on = 1, vsync off = 0
    ...
    QObject::connect(m_repaintTimer, SIGNAL(timeout()), this, SLOT(update()));
}

void GlWindow::initializeGL()
{
    initializeOpenGLFunctions();
    ...
    const double targetFrametime = 1000.0 / 120.0; //1000.0 / 120.0;
    m_repaintTimer->start(targetFrametime);
}

Everytime when gui gets updated i see a small stutter inside of my opengl window. QLineSeries updates seem to have a bigger effect, but even with a few QTableWidget changes i can see a small interupt in the rendering.

How do you normally update gui and opengl rendering at the same time?
Is there a way to the tell the gui it can render before or after opengl render frame is finished?