First, you might want to check to be sure you're not going to T5 if you are on Delta.
Mainly, though, the issue is not just traffic (once I waited in traffic on the way to FCO because of an accident on the divided highway, and the wait was almost an hour). The issue also is what's happening in the terminal, whether or not it is crowded with lots of passengers.
Yesterday, there was some kind of slow-down at FCO, intentional or not (I think it was intentional) at Passport Control. A delay of 45 minutes while the line did not move and just got longer. Then just one guy showed up to check passports. Really. And then you still aren't near the departure gate. (In T5, you aren't even at the departure terminal once you get through Passport Control.) So I would leave nothing less that 2 hours to go through all the steps and get to the gate, especially if you are hoping for some lounge time. Tried to leave 3 hours yesterday at FCO and T5, and ended up with a half hour of lounge time before making the move to the gate.