From the Toronto Pearson International Airport website (GTAA)
U.S. Customs and Border Protection: New entry measures are now in place for the United States.
Wait times are longer than usual at U.S. Customs and Border Protection (CBP) and Canadian Air Transport Security Association (CATSA) pre-board screening for flights before 10:00 a.m.
Please arrive at the airport 3 hours before your flight’s scheduled departure time when departing before 10:00 a.m. For flights departing after 10:00 a.m., please arrive at the airport 2 hours before your flight’s departure time.
For domestic flights (Canada), check In is at least 90 minutes prior to departure)