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.
