So if you are concerned about your data plan, performance, or security of your business correspondence stored on third party servers, you should consider one of the lightweight open source web alternatives for Slack:
Let's Chat - loads in ~5 seconds:
JabbR - loads in ~8 seconds:
Candy - loads in ~5 seconds:
Echoplexus - loads in ~6 seconds:
Shout - finally a web IRC client, loads in ~5 seconds:
Each of them is open source, so you can install it for free on your private server and keep your internal business communication secure.
UPDATE: There is a new project called Mattermost - check it out!