I took a similar flight. WS YVR-YYZ which arrived very early, maybe similar to the original poster's 5:53 am. There was plenty of time to eat breakfast. I even when to T1 to take a look at the departure hall then went back to T3 for a connecting flight to LGA. Everything was very fast, even the slow non-NEXUS line. I think I waited about 3 minutes for customs clearance though I did not have checked baggage, hence the freedom to explore another terminal.