The hills are one thing, and most of the roads I ride on are pretty wide and fast. If there's no bike lane, I'm typically just sticking to the right lane and letting traffic pass me. Philly, on the other hand, was full of narrow one-way streets with lots of tight squeezes and slow traffic. So if you could squeeze through tight spaces and ride between cars, you'd make it through the city much faster than a car could. I guess you get that in parts of LA too, but it doesn't seem as widespread in my experience.