last year and, as we approached, the traffic banked up alarmingly for about 500m or so. However once inside we moved very quickly through with no hold ups. Seems to be a quirk of tunnels as same thing often happens in the M5 tunnel in Sydney.
I think the traffic issues are mostly on weekends, especially during holiday periods (my guess is that a lot of Saturday's traffic was returning to Italy at the end of summer holidays; some might also have been en route to Monza for Sunday's Italian GP).
People get scared when it suddenly turns from day to night because they think the world has ended so it's time to stop the car.