Security/immigration clearances at YOW vary tremendously. Sometimes (even after Sept. 11, I've cleared quite quickly BUT bottlenecks/long lineups are possible/likely either at immigration or (last trip) security. On a few cases flights have been held because of many passengers in security/customs line holdups.
You'll probably be alright if you get there 2 hours ahead (because AC seems to hold flights if the lines are really bad) but you might even want to give it an extra half hour.
Waiting space in the transborder area is adequate most days, but not luxurious.