Apart from the install size which is still not optimal, it is very much possible to write an efficient Electron app, as evidenced by Discord and Element. That

Apart from the install size which is still not optimal, it is very much possible to write an efficient Electron app, as evidenced by Discord and Element. That Slack, the poster child of bad Electron apps performs so badly must therefore have different reasons. Exactly the same way you can write websites in a way that is more or less efficient you can write Electron apps of varying degrees of efficiency.