Slack is absolutely _unusable_ on a slow connection. It's not even just a matter of being patient, it flat out breaks with everything timing out.
I used to use the IRC gateway to maintain contact with some friends, and basically lost access to that social group when they closed down the IRC gateway.
IRC is far more inclusive, plus it's open and federated if you're into that.
Yes, IRC is much better, and is something I use. (IRC even works without specialized software, although it works better if you do have it (which isn't too complicated to write).)
I used to use the IRC gateway to maintain contact with some friends, and basically lost access to that social group when they closed down the IRC gateway.
IRC is far more inclusive, plus it's open and federated if you're into that.