The old Skype client using Qt was similar to a Slack using a classic GUI framework. Compared to Slack it was barebones. You had a list of conversations and a view for the conversations themselves. I'm not saying it's bad; I'd probably prefer it in fact. But it's not the same product as Slack. This comes back to my point about Electron vs. others, maybe the reason Electron apps are slow is because Electron apps typically have more functionality. The easiest way to make a program faster is to remove code.
> However, you cannot develop Chromium with Chromium
Why not? You can run Visual Studio Code in the browser and edit the Chromium source code. Perhaps, given sufficient memory, you can also run clang in the browser (WASM) so you can compile and link in Chromium too.
> Because it looks ugly (i.e. non-native) on pretty much every platform.
I'm not sure if you're talking about Electron or Qt because this statement applies equally well to both. Electron, Qt, Swing, JavaFX, Flutter all take the approach of drawing every pixel themselves, not using any "native" widgets rendered by the OS vendor.
Anecdotally all the Electron apps I've used have been beautiful whereas the Qt/GTK ones look stuck in the 90s.