Yep, exactly. I used to think Slack was the best thing since sliced bread, and now I keep pushing back to async communications like email to get my concentration and productivity back.
There's nothing about slack that makes it inherently more syncronous than email, as long as you switch off all its dumb notifications to make it work like IRC.
It feels, and this might just be my opinion, that Slack inherently has different response speed expectations than email (or other async comm methods) due to it being a chat client.
I still think Slack is the greatest thing since sliced bread, but I agree with you. If I need to send something non-time-sensitive to a specific person I will still go to email or another channel (1:1 meeting, Basecamp message, Trello card, ticket, etc) rather than sending an @mention.
If it's not specific to an individual, then I think dropping random things in channels works reasonably well asynchronously as it lets everyone decide when they have time to read the given channel, and of course it shows up in search for later reference. Slack stretches the boundaries of real-time communication quite a bit through clever design choices, but that's still its fundamental nature.
