I think that's a bit unfair to Slack. I've been using it for 5 years now and it's one of the most dependable apps I've ever used. When you consider how many messages and rich content it's handling, I think they are doing a fairly decent job. I wouldn't call it slow either. If anything my biggest beef with slack is sometimes the cross-device syncing isn't perfect. And I'm unable to use it offline at all.