You're right. QT is a good choice if you are willing to work within the bounds of the framework. QT is definitely not a good choice if you want to make a lot of customizations. If you want to make an app like Spotify, don't use QT.
Also, if I remember correctly there was a time when Spotify was written in Qt.
Lets say I want to control pan/tilt/zoom/focus/aperture/etc of a remote camera. If I ask lets say an expert in UI framework Z to do it, it will take them 10x longer to create a very painful experience using standard elements with poor input latency, so someone actually trying to setup a camera over/under shoots everything, but it technically "ticks every box". The path to create a better experience just isn't really there and it is difficult to undo/change all the boilerplate/structure, so version 1 isn't improved for years because it took so long to create the first iteration.
Maybe your definition of "fast" for a large text file is War and Peace, and mine is multiple 1/10/100 GB text files that you want to search/interact with in realtime at a keystroke level.
I've probably written 100+ completely different "very specific examples" in very different industries, because that's where you can create much better experiences.
Generally your expectations are based around what you get as standard from the library, but if you want to get a much better experience then it immediately becomes a lot more difficult.
At the end of the day, Qt can also be just a wrapper for your highly optimized engine - for example, NotepadNext[1] is using the very performant Scintilla engine while its UI is written in Qt. From my (unscientific) tests, it's even vastly faster than Sublime Text.
BTW, I'm not saying that rendering and creating your own UI is always a bad idea. Many people do it because it's fun and challenging, or to push the boundaries. That's what Vjekoslav Krajačić is doing with Disk Voyager - writing a file explorer in C from scratch[2][3]. But for many people, that's too much. I believe Qt C++ with QML is the best combo for most people, for most applications.
[1] https://github.com/dail8859/NotepadNext
[3]https://www.reddit.com/r/SideProject/comments/103b9fy/disk_v...
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.