Call stack:

0	QApplication::topLevelWidgets	qapplication.cpp	1762	0x642dfa1e	
1	QWindowsXPStylePrivate::winId	qwindowsxpstyle.cpp	387	0x64400cd2	
2	XPThemeData::handle	qwindowsxpstyle.cpp	203	0x644006a0	
3	XPThemeData::isValid	qwindowsxpstyle.cpp	188	0x6440078b	
4	QWindowsVistaStyle::drawPrimitive	qwindowsvistastyle.cpp	403	0x643e526b	
5	QProxyStyle::drawPrimitive	qproxystyle.cpp	191	0x643a97ea	
6	QStyleSheetStyle::drawPrimitive	qstylesheetstyle.cpp	4350	0x643b27ed	
7	QTreeView::drawBranches	qtreeview.cpp	1888	0x64648878	
8	QTreeView::drawRow	qtreeview.cpp	1755	0x64647ebb	
9	QTreeView::drawTree	qtreeview.cpp	1501	0x646473eb	

========================================================================================================

QWindowsVistaStyle::drawPrimitive	qwindowsvistastyle.cpp	403:
    case PE_IndicatorBranch:
        {
            XPThemeData theme(0, painter, QWindowsXPStylePrivate::TreeViewTheme);
            static int decoration_size = 0;
            if (d->initTreeViewTheming() && theme.isValid() && !decoration_size) {

========================================================================================================

XPThemeData::isValid	qwindowsxpstyle.cpp	188:
   return QWindowsXPStylePrivate::useXP() && theme >= 0 && handle()
   
========================================================================================================

XPThemeData::handle	qwindowsxpstyle.cpp	203
   htheme = QWindowsXPStylePrivate::createTheme(theme, QWindowsXPStylePrivate::winId(widget));
   
========================================================================================================

QWindowsXPStylePrivate::winId	qwindowsxpstyle.cpp	387
    if (widget)
        if (const HWND hwnd = QApplicationPrivate::getHWNDForWidget(const_cast<QWidget *>(widget)))
            return hwnd;

   foreach (const QWidget *toplevel, QApplication::topLevelWidgets())

========================================================================================================

QApplication::topLevelWidgets	qapplication.cpp	1762	
    QWidgetList all = allWidgets();

    for (QWidgetList::ConstIterator it = all.constBegin(); it != all.constEnd(); ++it) {
        QWidget *w = *it;
        if (w->isWindow() && w->windowType() != Qt::Desktop)
            list.append(w);
    }
    
========================================================================================================

Description:
drawPrimitive creates an instance of XPThemeData and then calls theme.isValid.
isValid calls handle.
handle calls winID.
winID calls topLevelWidgets because parameter widget is 0. Parameter widget is 0 because instance of XPThemeData was created with 0.
topLevelWidgets calls allWidgets and then iterates through all widgets.
Because we have more than 16000 widgets in our application topLevelWidgets takes many time.
For that reason painting of a branch in a QTreeView takes a lot of time, more than 0.4s.