When open/save file dialog is called the second time, it is super slow

The first call to file selection dialog (QFileDialog::getOpenFileName or QFileDialog::getSaveFileName), although a bit slow, executes normally. However subsequent calls become very slow, taking several seconds to draw themselves and respond to mouse clicks. Although the speed varies between executions (maximum goes into tens of seconds, in which case I kill the app), they always happen (are reproducible on my system). When I set breakpoints and divert execution from native into Qt dialogs, there are no problems.