Just flew YUL-VIE this week. I would say the main reason this flight is consistently delayed is that it doesn't board by jet bridge. We had a 30 min delay in boarding as we had to wait for PTV to shuttle us to the plane. Once boarded, then we waited 30 min for two missing cargo/luggage containers. Then, we had to wait for the ramp to push us back. I'm not sure why they couldn't just turn on the engines and go.

Touching down an hour late at 9:40, we ran from D to G (no transit security required) and got to the gate at 9:53 for a 9:55 departure. The boarding door was already closed. We were in Biz and first off the plane. They rebooked us for a flight that night, so we headed into the city of Vienna instead of waiting around.