I believe a lot of it is as Bjorn mentions, non-stock/missing stuff in the cars and smaller/clogged water passages. I know I ran better in the 60's before my arteries started getting clogged.

But, back in the 60's there were many less stop lights, stop signs, traffic jams and slow traffic. When I first started working (1969) I got to work 20 miles away in 25 minutes. When I drive to that same area today it takes about an hour.