The US immigration/Custom Opens exactly at 4:30am though the check-in counter is open much earlier than that. So once, you finished checking-in , you have to hold on your checked luggage(tagged) and wait in line for the immigration/custom to open.

My experience has been very quick (but I have Global Entry) . I believe there is also self-service kiosk within the immigration hall. Its the airport security after the immigration that has a bit of slow down. They only have a priority line for the Canada Nexus/Air Canada Premium. The rest is lumped into a general line.

Air Canada's MLL lounge opens at 4:45 am according to their website. However , this is one of the worse designed access lounge as there is no stairs. Do allow for more time to leave the lounge as the flight banks to US all departs around the same time.