Except for the startup time --- with which I can live with --- I never felt a difference in speed between native text editors ans VSCode. There is one, but I don't notice it. Human are much more slower than computers, so as long as it is not taking more time that I need to notice it, I don't care.
Nowadays my computer has enough memory so I can afford to not care about the 116M that VSCode is currently using, especially in regard to what my browser consumes, and in regards of the wide range of features it offers.
Battery could be problematic indeed, but before looking at my text editor, I would probably first switch from KDE to i3, and then use a lightweight web browser. This would impact it much more than my text editor.
> If a native editor just gave you a webview that can run JS extensions...
But then you start loosing all the benefits of faster/memory/battery points you mentioned.
Anyway, I'll happily try something like this if you develop it ;-)