Thanks. That confirms my hunch that the vertical padding is a bit greater in the mockup than in Slack. I think it would both look nicer and be more useful (ie. permit more conversation to be viewed at the same time) if it was a bit more compact in that regard.
It doesn't look to me like the cause of the extra vertical padding is the display of the avatars, but if that is the cause, perhaps showing avatars should be optional. Actually, that should probably be optional in any case. I expect some people will prefer the avatars, while others will not.
Thanks much!