CEO at Missive and proud developer of an electron based email client, I would love for you to try it and judge it's speed. It might change your opinion.
The problem is not when everything is written with web as the main central point, the problem is when you compare that with a previous version that was a desktop client first, then in a rush to make it cross-platform they fuck it up. Hence the hate for Electron, because 99% of Electron apps out there are like this.
I am a very happy Missive user and have no qualms about their desktop client. I kind of knew it was likely Electron, but it is so performant that it really didn't cross my mind that it was non-native. People bashing Electron apps is just a trope these days; like anything else, there are good ones and bad ones.