FCO has plenty of opportunities for intermittent bottlenecks. For instance, two weeks ago, it took 20 minutes to get through passport control - in the EU lane! They had exactly one agent working, and every single person in front of me let the guy have it. Took a while.
However, for the first time in my life, we landed at FCO and used a jet bridge to deplane. That probably saved me as much time over the usual bus ride as I lost in the passport control line.