Specifically, with regard to what we are discussing, it does as QT, technically, does a poorer job than the native libraries of the OS in "pushing" these pixels

Specifically, with regard to what we are discussing, it does as QT, technically, does a poorer job than the native libraries of the OS in "pushing" these pixels to the screen. In general too, when it comes to user interfaces - a lot of work has already been done to make an interface that is familiar and easy to use for the user. Often the recreation miss some design goals or usability features or even edge cases, and that ultimately provides a poor user experience. (For example, some native UIs allow a screen reader for blind users to read out the the UI elements - if your UI library doesn't support it, blind users won't be able to use applications built with such a library). QT does a great job for multi-platform application development. But like all other multi-platform toolkits, it does have its downsides.