Very slow to access QClipboard MIME data

There must be a bug either in QClipboard or QMimeData, as the following line

QApplication::clipboard()->mimeData()->data("text/plain");

takes ridiculously a lot of time (>100ms when just a single text line, roughly 20 characters, is in the clipboard).

I've uploaded an example project (just 3 files: main.cpp, QClipboardListener.h and QClipboardTest.pro). After building, run the application in a terminal, copy something into the clipboard and check the application output in the terminal, where it prints the time elapsed.

This very same bug is present in software that uses Qt 5 such as Telegram Desktop, where you can experience how the entire UI stalls when you paste an image with Ctrl+V. I've seen the UI blocked for longer than 10 seconds when the image size is 1920x1080.

Related to: https://bugreports.qt.io/browse/QTBUG-43497