Yes, you have to go through Canadian customs, and then you will end up near domestic arrivals.
You will have to go upstairs, near check in, and go through the regular security scan again. There is another security scan 1 level below the normal domestic security screening area in Terminal 1...Sometimes it's faster, and sometimes they have 1 or 2 lanes open and it takes forever....
I usually connect in toronto, and have no problems with going from plane, through customs, through security, and back to domestic departures in 45 mins. Had 75 mins last summer for a connection on a friday at 1pm....with checked bags, had time for a meal before my connection!