Qt list of images

I am looking for an efficient way to view a list of thumbnails of jpegs in Qt 5.8 (possibly several thousands).

My current approach is to use a QListWidget (see below, and this question). This turns out to be way too slow as it takes forever to assemble, even for a few images.

I am looking:

1. To understand why my current approach is so slow.
2. For a more efficient approach, which still requires only little code by relying as much as possible on Qt's features.

Current approach:

ui->listWidget->setViewMode  (QListWidget::IconMode);
ui->listWidget->setIconSize  (QSize(200,200)       );
ui->listWidget->setResizeMode(QListWidget::Adjust  );

for ( auto &i : files ) 
  ui->listWidget->addItem(new QListWidgetItem(QIcon(i),i));

(whereby files is of the std::vector<QString> type)

A quick solution might be to create a simple class that inherits from QListWidgetItem and fixes up the thumbnails on demand. You could try something like (note: untested)...

class thumbnail_item: public QListWidgetItem {
  using super = QListWidgetItem;
public:
  explicit thumbnail_item (const QString &filename)
    : super(filename)
    {}
  virtual QVariant data (int role) const override
    {
      if (role == Qt::DecorationRole) {
        QIcon i = super::icon();
        if (i.isNull()) {

          /*
           * The existing thumbnail (if any) is empty so create one
           * from the file name and use it -- remembering to call
           * setIcon so we don't need to go through all this again
           * next time.
           */
          i = QIcon(data(Qt::DisplayRole));
          setIcon(i);
        }
        return(i);
      }
      return(super::data(role));
    }
};

You still need the loop to create an item for each file but at least it defers the thumbnail construction.

A more elegant solution would probably involve using fully separated model and view objects with a custom proxy model performing the thumbnail generation and caching.