I was using Slack a few years ago and I didn't have this issue. Everyone who chose to use it had IRC experience, and so immediately treated it as a fancier IRC

I was using Slack a few years ago and I didn't have this issue. Everyone who chose to use it had IRC experience, and so immediately treated it as a fancier IRC. That includes a lack of expecting an immediate response. But I can definitely see how people who didn't grow up on freenode et al might come in with different expectations around Slack responsiveness.