Traffic out of downtown to YVR is extremely light, especially that time of day. If you the route, it should take you under 30 min. The hassle of a car return is like any other car return. If you take the Canada Line, it drops you off directly in front of Int'l/US departures.
The US Customs wait during the summer is ridiculous. At this time of year, almost non-existent. I'm just not sure how many US Customs agents are working that early in the morning as I generally use Nexus.