I guess your company has a lower message rate 
    than what I've seen. In current company there 
    are 30+ channels and a message arrives on average 
    about every minute.

Hopefully you're not being DM'd or @'d every minute.

Regular vanilla channel messages should never be expected to be synchronous. As you noted, that's completely unworkable.

DM's and mentions should be IMO be "semi-synchronous." Don't expect an immediate reply unless you've explicitly told the recipient that there's a blocker or there's some other time-critical situation.

Admittedly yes, that does require some attention on the recipient's part, because they have to scan the incoming message to see if it's critical. And some discipline on the sender's part. And even a few seconds of interruption can (worst-case) result in 20-30 minutes of lost productivity if one has to dump their entire mental stack.

So, yes it doesn't scale up endlessly. Multiple "semi-synchronous" mentions/DMs per hour would be a real attention drain. Possibly appropriate for managers, but a real deal breaker for individual contributors.

But at that point I would see it as an issue with the role or with the culture itself.

    The only solution is to switch it off entirely, only 
    check at specific times. But if the company culture 
    is to expect prompt response at all times, that's a 
    conflict.

How would you establish a healthy office communication culture? If everybody is always asynchronous, how could anybody be notified of time-critical stuff like blockers etc?

> Regular vanilla channel messages should never be expected to be synchronous. As you noted, that's completely unworkable.

It is unworkable! But becomes the expectation.

> How would you establish a healthy office communication culture?

The same way it has worked well for the ~30 years of my career before slack-obsession. Have something to discuss, put it on the calendar for the relevant people. For smaller things, send email and don't expect an immediate response. If things are truly on fire right now, make a phone call.

Curiously, in decades past we always used IRC which is basically same thing as slack. But IRC never grew this culture of obligated insta-response, it was more for chatter that one could read or ignore depending on how busy. Not sure why. Perhaps because slack is a paid product, it's now part of the business so everyone is expected to take it Very Seriously and be on it all the time.

You're either luckier than me, had the wisdom to choose better workplaces, or both.

    The same way it has worked well for the ~30 years of 
    my career before slack-obsession. Have something to 
    discuss, put it on the calendar for the relevant people.

In my ~20 years in the industry, instead of politely scheduling discussions for the future, people did one or both of the following:

1. Physically walked over to my desk and interrupted me in person

2. Sent me an email, often expecting a near-instant reply, requiring me to choose between "getting work done" and "monitoring my email."

This is why I feel that it's more about company culture than the tools themselves.

To be clear, a couple years back I did work at a company with a horrible Slack culture. I don't mean to defend Slack per se, and I definitely don't think it's the answer, but I don't think it's the problem either. Inconsiderate coworkers gonna be inconsiderate.

    Curiously, in decades past we always used IRC 
    which is basically same thing as slack. But 
    IRC never grew this culture of obligated 
    insta-response, it was more for chatter that 
    one could read or ignore depending on how busy.

Amen. This is why I strongly feel it's a culture issue and not a technology thing.

"Back in the day," generally it was the tech/engineer types using IRC. We all generally understood the concept of focus time and understood that IRC should not be used with the expectation of synchronous responses.

Now, with Slack, we have a lot of corporate/managerial/whatever types using an IRC-like tool, and they often just don't "get it." Much like my elderly father initially struggled to understand that text messaging wasn't something that guaranteed an instant response.