Try the following example to experience it.

#include <QtGui>
#include <QDebug>

int main(int argc, char *argv[])
{
QApplication app(argc, argv);

QStringListModel model;
QStringList list;
for (int i = 0; i < 20000; ++i)
list << QString::number;
model.setStringList(list);

QTreeView view;
view.setRootIsDecorated(true); // with no root decorated the speed is ok
view.setModel(&model);

view.show();

app.exec();
}