EffectiveDiscussions is an open source alternative, new this year 2017.
It has basic Slack features. And discussion forum features too, e.g. a recent topics list, and a popular topics list — so people know what everyone is talking about right now. And question-answers features, so you can use it for customer support too, and people would quickly find the best answers to their questions.
It's built with React.js, and the client side loaded-by-default bundle is only 140 kb (good for mobile phones). I haven't looked into memory usage yet. There's not yet any mobile client (the web version should be ok though). It's beta software. I'm developing it.
Chat example: effectivediscussions.org/-31/support-chat
Demo discussion: insightful.demo.ed.community/-7/how-do-i-get-myse… (there are more in that demo forum)
New unique features: effectivediscussions.org/-32/how-hacker-news-can-…
Nice looking topic list page (thumbnail images): beautiful.demo.ed.community/latest
It's free and open source; you can install it yourself. Or you can pay for hosting, so you won't need to install anything.