[the OlLine Rebel]

The breed was pretty well established as a breed in its own right by then, instead of a mutt constantly breeding back to horses unknown and arabs and such. That is my main point; other being the breed - indeed, the horse species - cannot improve too much. It is a logarithmic curve, and once you stabilize you are pretty much near the asymptote. Species can only ever get so much better; it’s built-in limits by God. You will never see a horse go 80 mph. So the difference from 1920 to 1970 to 2010 in theory is negligible.

[Nepeta]

Take a look at European time records. The winning times for the Epsom Derby and Arc de Triomphe continue to fall, while American record times have mostly stagnated, or in the case of the Belmont Stakes, begun to slide towards slower.

American "training" methods are to blame.