When comparing Electron and non-Electron, bear in mind that Chromium typically uses a fair bit more memory than Firefox; I’d estimate 20–40% as a rough figure, though it depends on all sorts of factors.

You pick Gmail as another example. Gmail is a memory hog and fairly slow. For comparison, Fastmail (which company I work for) tends to use 10–15MB in Firefox for slightly different functionality (no chat, and I estimate the Hangouts widget to be about half of Gmail’s bloat, but on the Fastmail side you can load the calendars, contacts and settings modules in the same document comfortably without breaking 20MB), and is a good deal snappier. It comes of careful engineering with performance as a priority, and a small team.

Slack is simply atrocious in these regards. Painfully slow, utterly greedy for memory. I often run it in Firefox rather than standalone to keep its memory footprint down. Whichever way I do it I regularly find it to have crept up over a gigabyte of memory, on my single workspace.