This definitely isn't slack-specific, and it drives me nuts as well. I've seen the same in hipchat and just about any real-time chat client in the past. From

This definitely isn't slack-specific, and it drives me nuts as well. I've seen the same in hipchat and just about any real-time chat client in the past. From what I've been able to tell, it's conversational, where the person types as they would speak - at the speed of thought, and others can potentially jump in (as is possible in normal conversation). This brain-dump method also seems to avoid the "x is typing..." (as fluxquanta mentioned) pause, which, if poorly implemented in some chat clients, will show "x is typing..." even if x stopped typing and stepped away. That's just as infuriating - especially if you actually need to know what x was typing. Personally, I still prefer email. There are very few times that real-time communication has been better for me, personally, than a well-thought-out email and response chain, which I can search and reference at another time. The more a team I'm working with leans toward real-time chat, the more I find myself logging out of chat to get things done and then reviewing and responding at length when I can focus on doing so.