We went through FCO on a United flight to IAD (and beyond). Other than being busy at the United counter, we found it to be basically pretty efficient. We got there slightly over three hours early, checked the bags and worked our way through passport control, etc., and had plenty of time for a coffee and croissant prior to boarding. (Compare and contrast to IAD, where we got to passport control right at shift change and within minutes of two other large international flights -- thank goodness for Global Entry! our line was about 25 people instead of the 600 or so in the regular line -- and it took a long time for the luggage to come through to be rebooked.)