Slack, like all text messaging is an asynchronous messaging tool. Of course it's horrible using it synchronously the way you describe but that's not the way it's meant to be used or is used in my company. I often miss direct mentions on slack because its mobile notifications are almost non-existent or because I'm working on something else. Someone who is too stupid to understand the nature of asynchronous communication and calls out people for not responding right away is an idiot and an asshole whether it's a manager at work or a so called "friend" (no one who makes synchronous demands on one's time to text chat can be called a friend). Slack used properly is about as far from a synchronous meeting as you can imagine. A boss that demands constant synchronous slack communication is a sign to look for a new job.