It’s not the dozen websockets in a literal sense that are the problem; they’re more an especially-visible consequence of their design choices that is easily noticeable during a design audit—a “canary” that shows the flaws of their model.
Slack has decided that each team workspace must be firewalled off from the others: such that it needs its own websocket, its own client-side sync session, its own open channels, etc. This is because Slack was first-and-foremost a web-app, and in the Slack web-app, you’d just have one browser tab per Slack team. Slack built up its infra assuming this “one tab per workspace” model, and it crept into their backend API design, and now they’re kind of stuck with it. So now, even in the native mobile app (let alone the desktop Electron app), when you have 12 Slack teams open, you have basically 12 copies of the app’s view-controllers open and idling in the background, 12 background sync controllers, etc. In the case of the Electron client, that’s also 12 renderer processes, just as if you had 12 literal browser tabs open!
You might not notice this at first, because Slack won’t initialize all those resource for a workspace’s “tab”-equivalent until you actually focus the given workspace at least once. But if you just skim through all those workspaces once in the morning, and leave Slack open, it’ll be hogging those resources all day.
(And, on mobile, that’s especially a concern, because despite what you say, web-sockets themselves have heartbeats, and 12x the heartbeats is 12x the reasons for your phone’s modem to wake up. If you’re ever wondering why the part of your phone around the antenna is getting hot even when you’re not actively using data—it’s probably that you have several active Slack workspaces in a live Slack app-container.)