I bought Cometchat for my forum.
And I must say, It works really great and fast!
Lots of options, Addons, Theme's ...
Yeah I keep Mchat! Just bought this for private chat.
The social network mod with almost the same chat is blowing up my CPU usage..
This one from cometchat 20 to 35 % ..
Preview: http://www.youtube.com/watch?v=IRA8IXFkJek
And if your server can't handle the load, You can use another domain / server to transfer the load..
http://www.cometchat.com/cometservice