Slack on the desktop is a hunk of garbage. Sign in to even 3 slacks and it will eat almost a gig of ram and 5% CPU to... display v2014 irc chat. They really should be ashamed of themselves. I have to remember to kill it whenever I'm on battery.
Both Rambox and (as I understand it) Franz just load the web interfaces of the services you configure, so performance wouldn't be better than Chromium, since that's what Electron uses. The added value is in the wrapper, which helps to keep everything organized, lets you open and close all services at once (by opening/closing Rambox), and provides a global 'do not disturb' mode for notifications.
To display a chat. Or four.