There is evidence that indicates that kids train faster in cloth.

The theory is that cloth allows a child to feel when they are wet. It allows them to associate wetness with the feelings of elimination. Kids in disposables can't feel when they are wet, and so they don't learn to associate these feelings until much later.

An article from the Wall Street Journal presented statistics that kids in cloth train at 24-30 months, while kids in disposables train around 36-40 months.

Here are other interesting statistics about toilet training trends:

In 1957, 92% percent of children were toilet trained by 18 months.
Cloth diaper usage at this time was 100%.

Today, only 25% of kids are toilet trained by 18 months. More than half of kids (60%) train at 36 months. There are about 2% of children who aren't trained by age 48 months.
Cloth diaper usage today is about 10%.