FWIW I use Zulip every day -- I work on Zulip, and naturally we use it among ourselves -- and I never touch the Electron client, because I prefer the experience of a browser tab.
Chrome's task manager (shift-esc) reports that tab is using 281M right now. Less than Gmail, or some old GitHub tabs (>1G, yikes!), or a freshly-started VS Code (... oh wow, or my Emacs which is at 337M. Wasn't expecting that.) Not sure how that compares to the Electron app.
As another reply pointed out, there is also a terminal client! You might enjoy trying it: https://github.com/zulip/zulip-terminal
I just migrated off of and removed Atom, GitKraken and Github Desktop to only use VS Code but I don't think I am able to open Chrome or Firefox without it swapping out to my SSD. I have to kill two or three other apps to recover my RAM usage just to stop the constant freezing when using either browser. After sometime browsing the internet, I see a list of swap-files everywhere and eventually I run out of space.
So when I see another Electron app to use instead of Slack, I usually avoid it and I try to find another usable alternative if I can.
I really want to like Zulip Chat. It has everything all the alternatives have + self-hosting, but as soon as they switched to their Electron client, it has become yet another resource hog that I have to put up with.
I already have to put up with 10+ electron apps slowing down my Macbook. I shouldn't have to make space for another to do the same.