Training Speed

[Argent]

Are horses supposed to learn skills at different rates, or is the difference in performance ability determined primarily by their breeder report, i.e. if there are two horses with equal training, the one with the better breeder report for that skill will perform better.

[larissar]

if there are two horses with equal training, the one with the better breeder report for that skill will perform better.

This part is true.
Natural ability is expressed via the Breeders Report. Training just brings out the horses natural ability. So 100% of a trained stat means that horse is able to use 100% of his natural ability.