[simonh]

Qt is fantastic, I've used it with Python using the semi-commercial PyQT and open source PySide which are largely mutually compatible. The example projects that come with it are a great way to get started and the documentation is world class. I found it much easier to work with than the native Windows options 10 years ago, but haven't really looked at anything from MS since.

[pfundstein]

I've often found Qt GUIs to be bloated and slow. Not sure whether that's brought about by Qt inherent overheads or bad design by those using Qt.

[simonh]

That may be so, but I've used very performant software that used Qt. The desktop version of Google Earth was a Qt application and was amazing. The whole KDE desktop is Qt, as are Ableton Live, Mathematica, and several Adobe and Autodesk apps. If a Qt app has performance issues it's unlikely to be down to the UI layer, unless they're doing something particularly weird. Having said that I have no experience of QML or how it performs.