Performance issues when painting text on custom item delegates in Qt C++

Question

Goal: Creating an item delegate with custom textual content to be used in QListView.

Problem: Drawing text with QPainter in a reimplementation of the paint() method of a QAbstractItemDelegate's subclass is noticably slower than drawing shapes and pixmaps. Changing the base class to QStyledItemDelegate does not improve the speed.

Setup: Qt 5.9.1, MSVC 2017, tested on Windows 7/10

Pre-research: A similar bug is reported here, however in this particular case the performance issue exists even if QPainter::setFont() is not used. The Qt examples on item delegates do not help much, as they show how to draw controls, not just text.

Example: The example given below illustrates the problem. After showing the application's window, the content of the QListView is shown with a slight, but noticable delay. In some cases this delay lasts up to several seconds. When either of view->setItemDelegate(new Delegate(this)); or painter->drawText(0, option.rect.y() + 18, index.data().toString()); are commented there is no observable delay between showing the window and the content of the QListView.

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QListView>
#include <QStandardItemModel>
#include "Delegate.h"

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
};

#endif // MAINWINDOW_H

MainWindow.cpp

#include "MainWindow.h"

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
    QStandardItemModel *model = new QStandardItemModel(this);
    QListView *view = new QListView(this);

    view->setModel(model);
    view->setItemDelegate(new Delegate(this));

    for (int n = 0; n < 100; n++) { model->appendRow(new QStandardItem("Test " +  QString::number(n))); }

    setCentralWidget(view);
}

Delegate.h

#ifndef DELEGATE_H
#define DELEGATE_H

#include <QAbstractItemDelegate>
#include <QPainter>

class Delegate : public QAbstractItemDelegate
{
    Q_OBJECT
public:
    explicit Delegate(QObject *parent = nullptr);

    void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const;
    QSize sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const;
};

#endif // DELEGATE_H

Delegate.cpp

#include "Delegate.h"

Delegate::Delegate(QObject *parent) : QAbstractItemDelegate(parent)
{

}

void Delegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    painter->drawText(0, option.rect.y() + 18, index.data().toString());
    painter->drawLine(0, option.rect.y() + 19, option.rect.width(), option.rect.y() + 19);
}

QSize Delegate::sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    return QSize(0, 20);
}

Answer 1

For now I've created a bug report. I will update this post with any further info I receive.