This is my route so I will comment.

Depending on the flight and the incoming traffic, clearing customs in SEA can be very quick or can take as much as an hour. I have taken the 4:35 pm YYJ which arrived at SEA at 5:05 last fall, cleared customs, took the bus to C termiinal and managed to standby for a 5:30 flight. I find the 4:35 and 7:30 flights are the best for customs.

Two weeks ago, we took the 2 pm flight, and managed to get stuck behind a flight from Asia, so it took almost 40 minutes to clear customs, and then were behind the Asia passengers, so the second screening section took another 30 minutes. (then add shuttle time to C or D gates) you can see that it can take as much as 1:25 minutes. That was a record, and it usually is pretty swift.

In the spring I took Grandma, and the staff was very good at SEA, they met the plane with a chair, and wheeled us through the back way to customs and waited for us all the way through customs and on to the subway to D gates. Because we were on the wheel chair we ended up getting in the front of the line and cleared customs and made it to the gate in about 25 minutes all in.

Hope this helps.