Half my time in Slack is just banter and gifs, the other half is skimming discussions that would've been a meeting otherwise. I think the time I waste chit chatting about cancels out the time I've saved from emails, meetings, briefings, and forced context switching from shoulder tapping. From a productively standpoint, it's neither a gain or loss. But the real win for Slack (but also shared with email and tickets) is the journaling aspect of chat, which is typically leaps and bounds ahead of meetings without minutes or hallway discussions.
I've also noticed that any channel that has more than ~10 active participants quickly turns off topic or impossible to follow. That I can blame on Slack's awful implementation of threading which Zulip seems to do better at.
The first problem is actually easy to fix -- make it a rule that anyone that sends unrelated gifs/makes funny banter gets to buy lunch for the others ;-) (or makes a charitable donation in proportion to the number of people -- if the users are geographically dispersed). And enforce it strictly. You'll find that your slack channel is automatically devoid of unnecessary chatter...something about having to buy lunch for colleagues brings out the miser in them :-).
I've also noticed that any channel that has more than ~10 active participants quickly turns off topic or impossible to follow. That I can blame on Slack's awful implementation of threading which Zulip seems to do better at.