the speed of the chat depends on your server, it is AJAX and fully configurable.
It will be optimized for XF also, it's possible the first version will just be a direct port so people have a shoutbox to run with off the bat, but certainly after that it would be tailored to XF - all of our products (lite and pro) get updated several times a year.
As Floris pointed out, some people prefer IRC to a shoutbox, though it can be difficult/impossible getting the same level of integration.
Our shoutbox has a number of features to add to a members posting experience rather than detract (notifications of replies to threads in the box amongst other things)
xD I feel this is getting slightly off topic though, so to drag it back - there will 100% without a doubt be at least one fully featured shoutbox on XF, and if another 2 or 3 don't pop up in the months after released i will be extremely surprised =)