slow QTreeWidget.clear when the items are expanded

# Python
import sys
from PySide6.QtWidgets import QApplication, QMainWindow, QWidget, QVBoxLayout, QTreeWidget, QTreeWidgetItem, QPushButton, QSpinBox, QCheckBox
from PySide6.QtGui import QPalette, QColor


class MainWindow(QMainWindow):

    def __init__(self):
        super(MainWindow, self).__init__()

        self.setWindowTitle("My App")

        layout = QVBoxLayout()

        self.t = QTreeWidget()

        b = QPushButton('clear tree')
        b.clicked.connect(self.clr)

        self.c = QCheckBox('Items collapsed')

        self.t.setHeaderLabels(
            [
                "Name",
                "Group",
                "Index",
                "Unit",
                "Source name",
                "Source path",
                "Comment",
            ]
        )

        self.n = n = QSpinBox()
        n.setMaximum(1000000)
        n.setValue(1000)

        a = QPushButton('Add items')
        a.clicked.connect(self.add)

        layout.addWidget(self.t)
        layout.addWidget(n)
        layout.addWidget(a)
        layout.addWidget(self.c)
        layout.addWidget(b)

        widget = QWidget()
        widget.setLayout(layout)
        self.setCentralWidget(widget)

    def add(self):
        count = self.n.value()
        if count:
            top_level = [QTreeWidgetItem([
                f"Name{i}",
                f"Group{i}",
                f"Index{i}",
                f"Unit{i}",
                f"Source name{i}",
                f"Source path{i}",
                f"Comment{i}",
            ]) for i in range(count)]
            for item in top_level:
                item.addChildren([QTreeWidgetItem([
                    f"_Name{i}",
                    f"_Group{i}",
                    f"_Index{i}",
                    f"_Unit{i}",
                    f"_Source name{i}",
                    f"_Source path{i}",
                    f"_Comment{i}",
                ]) for i in range(3)])

            self.t.addTopLevelItems(top_level)
            self.t.setSortingEnabled(True)

    def clr(self):
        from time import perf_counter
        count = self.t.topLevelItemCount()
        all_collapsed = self.c.isChecked()
        if all_collapsed:
            self.t.collapseAll()
        else:
            self.t.expandAll()
        t1 = perf_counter()
        self.t.clear()
        t2 = perf_counter()
        print(f'{all_collapsed=} {count=} took {t2-t1}')


app = QApplication(sys.argv)

window = MainWindow()
window.show()

app.exec_() 

// C++
#include <QApplication>
#include <QMainWindow>
#include <QWidget>
#include <QVBoxLayout>
#include <QTreeWidget>
#include <QTreeWidgetItem>
#include <QPushButton>
#include <QSpinBox>
#include <QCheckBox>class MainWindow : public QMainWindow
{
    Q_OBJECTpublic:
    MainWindow()
    {
        setWindowTitle("My App");        QVBoxLayout* layout = new QVBoxLayout;        treeWidget = new QTreeWidget;        QPushButton* clearButton = new QPushButton("Clear tree");
        connect(clearButton, &QPushButton::clicked, this, &MainWindow::clearTree);        checkBox = new QCheckBox("Items collapsed");        treeWidget->setHeaderLabels({
            "Name",
            "Group",
            "Index",
            "Unit",
            "Source name",
            "Source path",
            "Comment"
        });        spinBox = new QSpinBox;
        spinBox->setMaximum(1000000);
        spinBox->setValue(1000);        QPushButton* addButton = new QPushButton("Add items");
        connect(addButton, &QPushButton::clicked, this, &MainWindow::addItems);        layout->addWidget(treeWidget);
        layout->addWidget(spinBox);
        layout->addWidget(addButton);
        layout->addWidget(checkBox);
        layout->addWidget(clearButton);        QWidget* centralWidget = new QWidget;
        centralWidget->setLayout(layout);
        setCentralWidget(centralWidget);
    }public slots:
    void addItems()
    {
        int count = spinBox->value();
        if (count > 0)
        {
            QList<QTreeWidgetItem*> topItems;
            for (int i = 0; i < count; ++i)
            {
                QTreeWidgetItem* item = new QTreeWidgetItem({
                    QString("Name%1").arg(i),
                    QString("Group%1").arg(i),
                    QString("Index%1").arg(i),
                    QString("Unit%1").arg(i),
                    QString("Source name%1").arg(i),
                    QString("Source path%1").arg(i),
                    QString("Comment%1").arg(i)
                });                for (int j = 0; j < 3; ++j)
                {
                    item->addChild(new QTreeWidgetItem({
                        QString("_Name%1").arg(j),
                        QString("_Group%1").arg(j),
                        QString("_Index%1").arg(j),
                        QString("_Unit%1").arg(j),
                        QString("_Source name%1").arg(j),
                        QString("_Source path%1").arg(j),
                        QString("_Comment%1").arg(j)
                    }));
                }                topItems.append(item);
            }            treeWidget->addTopLevelItems(topItems);
            treeWidget->setSortingEnabled(true);
        }
    }    void clearTree()
    {
        bool allCollapsed = checkBox->isChecked();
        if (allCollapsed)
            treeWidget->collapseAll();
        else
            treeWidget->expandAll();        treeWidget->clear();
    }private:
    QTreeWidget* treeWidget;
    QSpinBox* spinBox;
    QCheckBox* checkBox;
};int main(int argc, char *argv[])
{
    QApplication app(argc, argv);    MainWindow window;
    window.show();    return app.exec();
}
 

all_collapsed=True count=100 took 0.0008545000018784776
all_collapsed=True count=1000 took 0.00891409999167081
all_collapsed=True count=10000 took 0.09732249999069609
all_collapsed=False count=100 took 0.0017668999935267493
all_collapsed=False count=1000 took 0.10975079999479931
all_collapsed=False count=10000 took 17.177133699995466