Understanding why these few lines are slow

[Lykurg]

and i would add remove the heap allocations

Have you? You don't see it in your code but:

void QListWidget::insertItem(int row, const QString &label)
{
    Q_D(QListWidget);
    d->listModel()->insert(row, new QListWidgetItem(label));
}

void QListWidget::insertItem(int row, const QString &label)
{
    Q_D(QListWidget);
    d->listModel()->insert(row, new QListWidgetItem(label));
}

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And as you might guess, addItem is just a convenient function for insertItem, so you could at least save one function call by using insertItem direct. But the real problems, to point them out, are in the takeAt() like, the "incorrect" use of mid(), the loop itselfs...