Just a few points to add:

DL flies into YYZ T3 which is the same terminal as South Asian, Caribbean and vacation charter airlines. It is realtively fast for the Dl flight are no more than 50 passengers unless you run into one ore more of these large jets landing at the same time. It can get very slow due to improperly filled out arrival time and additional screening for incoming passengers.

Flying out DL counter is the closest to the US CBP area. it is not a long walk. The lineup can be long during rush hour for US flights. Mornings before 9 and Th/F evenings. Monday morning is the worst. The slowness of security check by CATSA is the most time consuming. 2+ hrs are not unheard of for these checks on a Monday morning. I have been flying out of BUF if I need to be in US on Mondays.

Make sure you have the forms filled out properly. It is very straight forward. You will get a Canada Custom declaration form either at gate in ATL or on your ATL YYZ flight that you can fill in before landing. If they don't have any they are at Canada entry point but it is a counter and can be a madhouse at the wrong time.
Departing at YYZ you will have to fill in the US custom declaration form, They are at the DL check in counter and is blue.