Welcome to FT
IanFromHKG!
You should look at this thread in the Midwest - Chicago forum ...
http://www.flyertalk.com/forum/chica...rd-thread.html
I go through immigration and customs 4 to 5 times a year at ORD. The lines at passport control can be long sometimes. But, most people get through before their bags arrive on the carousel. The bags are the slow point at ORD.
Once you leave customs and go through the exit doors, you will find airline check-in counters for connecting international passengers. Since I live in the Chicago metro area, I have never used them. But they have never seen too congested.
I would ask the moderators to move this to the Chicago forum and you will get better answers there.