Terrible painting performance on iPhone, why?

#include "drdrawview.h"

DRDrawView::DRDrawView(QWidget *parent) : QGLWidget(parent), image(1024, 1536, QImage::Format_ARGB32_Premultiplied)
{
    pen.setColor(Qt::black);
    pen.setWidth(20 * devicePixelRatio());
    pen.setCapStyle(Qt::RoundCap);
    pen.setJoinStyle(Qt::RoundJoin);
    setAutoFillBackground(false);
    setAttribute(Qt::WA_OpaquePaintEvent, true);
    setAttribute(Qt::WA_NoSystemBackground, true);
    clearScreen();
}

DRDrawView::~DRDrawView()
{

}

void DRDrawView::save( const QString &filename, const QString &format )
{

}

void DRDrawView::clearScreen()
{
    QMutexLocker l(&bufferMutex);
    memset(image.bits(), 0xFF, image.byteCount());
    update();
}

void DRDrawView::mousePressEvent( QMouseEvent *e )
{
    mousePressed = true;
}

void DRDrawView::mouseReleaseEvent( QMouseEvent * )
{
    QMutexLocker l(&bufferMutex);
    mousePressed = false;
    polyline.clear();
}

void DRDrawView::mouseMoveEvent( QMouseEvent *e )
{
    if ( mousePressed )
    {
        QMutexLocker l(&bufferMutex);
        QPoint p = e->pos();
        p.setX(p.x() * devicePixelRatio());
        p.setY(p.y() * devicePixelRatio());

        polyline.push_front(p);
        if (polyline.count() == 4)
        {
            polyline.pop_back();
            QPainter painter(&image);
            painter.setRenderHint(QPainter::Antialiasing);
            painter.setPen(pen);
            painter.drawPolyline(polyline);
            QRect r = polyline.boundingRect();

            r.setWidth(r.width() / devicePixelRatio());
            r.setHeight(r.height() / devicePixelRatio());
            r.setX(r.x() / devicePixelRatio());
            r.setY(r.y() / devicePixelRatio());

            r.setLeft( r.left() - penWidth() );
            r.setTop( r.top() - penWidth() );
            r.setRight( r.right() + penWidth() );
            r.setBottom( r.bottom() + penWidth() );

            r = r.normalized();
            qDebug() << "DRAW RECT: " << devicePixelRatio() << r.x() << " " << r.y() << " " << r.width() << " " << r.height();

            update(r);
        }
    }
}

void DRDrawView::resizeEvent( QResizeEvent *e )
{
    QWidget::resizeEvent( e );

    //image = QImage(width() * devicePixelRatio(), height() * devicePixelRatio(), QImage::Format_ARGB32_Premultiplied);
    //clearScreen();
}

void DRDrawView::paintEvent( QPaintEvent *e )
{
    //QGLWidget::paintEvent( e );

    QMutexLocker l(&bufferMutex);
    QVector<QRect> rects = e->region().rects();
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);

    for (int i = 0; i < rects.count(); i++)
    {
        QRect r = rects[i];
        QRect r2 = r;
        r2.setWidth(r2.width() * devicePixelRatio());
        r2.setHeight(r2.height() * devicePixelRatio());
        r2.setX(r2.x() * devicePixelRatio());
        r2.setY(r2.y() * devicePixelRatio());

        //painter.drawImage(r.x(), r.y(), &image, r.x(), r.y(), r.width(), r.height(), Qt::AutoColor);
        painter.drawImage(r, image, r2);

        qDebug() << "UPDATE RECT: " << devicePixelRatio() << r.x() << " " << r.y() << " " << r.width() << " " << r.height();
    }
}

/*
Logging:
UPDATE RECT:  2 0   0   320   568
UPDATE RECT:  2 0   0   320   568
DRAW RECT:  2 128   159   266   302
UPDATE RECT:  2 0   0   320   568
DRAW RECT:  2 87   171   255   398
UPDATE RECT:  2 0   0   320   568
DRAW RECT:  2 75   182   248   431
DRAW RECT:  2 73   278   207   434
UPDATE RECT:  2 0   0   320   568
*/