It's because of work done on the main thread when it should be done using Workers. More about the lack of proficiency with regard to understanding UI applications themselves. Block the event loop in another language and you'll also get a laggy irresponsive UI.

There are probably a lot of different reasons why Electron apps can get laggy and unresponsive when sloppily written.

However, I notice a significant difference in responsiveness between VS Code and Sublime Text -- enough that I changed my workflow back to Sublime Text because the very slight latency difference annoyed me. So I do think there is a baseline difference between the frameworks used by those two apps that no amount of optimization can overcome. It's sort of like the acceleration difference between a truck and a sedan: sure, powerful trucks can sometimes out-accelerate anemic sedans. But if you put a similar drivetrain in both vehicles, the vehicle with less mass (or memory footprint, in the framework analogy) is going to win.