HI,
Another hint is to tune the axles, which are nails. Some nails have
little "ears" caused by the stamping process that formed the head of the
nail. File these down and smooth and polish the axle. Check the side
of the wheels to see that they are smooth.
As mentioned by others, not tracking straight will scruff off a lot of
energy.
My introductory physics logic says the weight should not matter, but
my gut, is pretty sure that "secondary" effects (like even slight
tracking problems) will favor the heavier cars.