Security yes and USA immigrations and customs due to pre clearance of transborder flights.

I'm not sure whether there's a transit bypass at YYZ that enables you to avoid Canadian immigration and customs. When I tried to do this a long time ago with PMNW, I did need to enter Canada formally and then go landside to get to the terminal area for the transborder flights. In the other direction, also with PMNW, a colleague of mine was trapped in a holding pen while waiting for a TATL flight and could not access the lounge KLM uses. However, I think the airport has been modified/renovated since then, so the situation now could be different.

I wouldn't try this because I've experienced long lines for both preclearance and security when attempting to leave Canada for the USA. IME if you attempt to check in for a transborder flight at T-60, you risk missing your plane due to these long lines and many transborder flights leaving around the same time.