> And if someone built slow Electron app, I'm sure that he would build slow Qt app (not sure if Qt considered slim nowadays). It comes either from lack of experience or lack of focus on performance. And that does not depend on stack.

No. Literall every Electron chat app (Signal, Discord, Element, Slack, etc) is slow, laggy and unresponsive, while every Qt chat app (Telegram, Ripcord, Nheko) is blazing fast.