qml with QQuickPaintedItem will slow down the ui operation

[QtTester]

hey, guys,
I want to display camera picture throuth QML with QQuickPaintedItem, but I found a few problem.
the update timer (about 45ms to 33ms):

void MainWindow::onLoadTimer()
{
    static int idx = 1;

    QString file = QString("%1.jpg").arg(idx,2,10,QChar('0'));
    QImage img(file);

    m_iv->updateImage(img);
    ++idx;
    if(idx > FPS)
        idx = 1;
}

the update func:

void CamImageItem::updateImage(const QImage &img)
{
    if(img.isNull()){
        return;
    }
    m_bgImg = std::move(img);

    if(m_lastImgHeight!=img.height() || m_lastImgWidth!=img.width()){
        auto map = fitToShowAll(m_lastWinWidth,m_lastWinHeight);

        emit sigImageChanged(map);
    }
    update();
}

Solution1, only draw qimage in qwidget's paintEvent, the cpu usage is up to 15%, but draging window seems no affection.
Solution2, use QQuickPaintedItem class :

class CamImageItem : public QQuickPaintedItem
{
    Q_OBJECT
...
protected:
    void paint(QPainter *painter)override;
...
}

void CamImageItem::paint(QPainter *painter)
{
    auto *img = &m_bgImg;

    // image and cross
    painter->drawImage(0,0,*img);
    painter->setPen(QPen(Qt::red,1));
    painter->drawLine(0,img->height()/2.0,img->width(),img->height()/2.0);
    painter->drawLine(img->width()/2.0,0,img->width()/2.0,img->height());
}

the cpu usage is up to 16%, GPU up to 10~20%. but draging window seems slow down:

or see it in : https://ibb.co/G4vsQFS1

Solution3. donot draw image in paint Event, still the cpu's usage is up to 15%:

void CamImageItem::paint(QPainter *painter)
{
    auto *img = &m_bgImg;

    // do not draw image still occupy cpu 15% and gpu to 10%
    // painter->drawImage(0,0,*img);
    painter->setPen(QPen(Qt::red,1));
    painter->drawLine(0,img->height()/2.0,img->width(),img->height()/2.0);
    painter->drawLine(img->width()/2.0,0,img->width()/2.0,img->height());
}

so How do i optimise the draging perfermance or the cpu usage? thanks.