Yup, another good example of this is Hyper vs. Terminus. Both Electron apps, but Terminus is way more performant and less memory hungry. I've taken that (along with Slack vs. pretty much every other Electron app) to mean that yes you're going to get some sort of inherent performance hit going Electron over native, but a huge performance hit can arise from a crappy architecture/implementation. For instance, doesn't Slack use a different webview for each channel?

The MS team worked very hard to make it snappy; e.g. it doesn't dump the entire file into the DOM if it is big (but rather materializes it as needed, and removes sections not viewed).

That's the kind of tricks that the DOM (so web and also Electron) were supposed to make irrelevant.

Write an app "naturally" with Electron, and it will likely be slow because of the DOM <-> data impedance mismatch. That's what slack is guilty of, but the ability to do so is Electron's claim to fame.