In the following example, each keystroke in the lineedit takes about 600ms before the UI reacts. Using QCompleter::PopupCompletion is much faster, as there are viewer items that the view has to layout.

#include <QtGui>

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

QStringList items;
for (int i = 1; i < 9999; ++i)
items << QString("%1").arg;

QWidget widget;
QComboBox combo;
combo.setEditable(true);
combo.setInsertPolicy(QComboBox::NoInsert);
combo.completer()->setCompletionMode(QCompleter::UnfilteredPopupCompletion);
combo.addItems(items);

QHBoxLayout* hbox= new QHBoxLayout(&widget);
hbox->addWidget(&combo);

widget.show();
return app.exec();
}