If everything you do stays in the rails of QT you're going to be fine. But you try to do something simple like load a 2GB file and everything starts to fall to

If everything you do stays in the rails of QT you're going to be fine. But you try to do something simple like load a 2GB file and everything starts to fall to pieces, then you're going to assume the people that wrote this are super clever and that a 2GB file is too complicated, too niche, too hard of a problem, it needs to be "highly optimised", etc. The reality is QT/whatever program/framework is doing 100 things you don't care about when loading/rendering a file. If we only care about 1 thing we can do that much better because we don't care about the 100 other things our naïve code we wrote in 5 minutes outperforms the standard element by a factor of 1,000.