As you have pointed out, QML is buggy. Chromium's rendering engine is probably the most stable and polished GUI toolkit there is, not to mention a cross-platform one too. Throughout the last 10 years I only had to deal with 2 Chromium bugs and they were very minor. Well-written JavaScript is fast and the machines are getting faster every year. It does not take much real time computation to provide a UI for a desktop app, it's not a video game. And many of the those things that are real time, like the caret in the text editor or hover states are implemented in native code by the web browser, with no JS interaction. I agree though that a block editor is a little more real time than the average UI.
The key word is well-written JavaScript. What is the most popular state management framework? Redux, possibly. What is the most inefficient state management framework? Also Redux. With Redux, if you have an app that displays a timer that updates every second, every subscription to any piece of the state throughout the entire app will trigger. I'm not sure if the app used Redux, but I used to use a time tracker app that would use 30% of my CPU when idle (I since moved to a CLI C++ solution and it is so much faster, but that does not mean a decent time tracker could not be built with web technologies). So if Redux is the most popular framework, you can see just how little the average web dev cares about writing apps that are not slow resource hogs.
> Also, all web apps couldn't handle loading a very large text file (they were all hanging).
Could it be that QT has some optimisation technique to not render all those lines out of view? I.e. if you have a huge file that can still be loaded to RAM, C++ won't sweat it, but is it actually getting all rendered at the same time in a savvy implementation, whether at the level of the app or the framework? Probably not. On the other hand, the textarea element or a contentEditable div just was not made for something like this. It could still be developed by implementing a custom element / component that loads the text dynamically while scrolling. If it's too much for JS to hold, it could use WASM or another process and pass it with IPC. It is definitely possible to write an Electron-based text editor that can open a 1 GB text file efficiently, it's just not out of the box experience and most people do not think there's a need for such a use case.