It is easy to assume that breeders continue to produce faster and faster horses each year. The truth is that Kentucky Derby winners have been slowing down for some time. As you can see, since the 1960’s the winning run has trended (ever so slightly) slower. Of course, the differences in speed we are talking about are far to minute to be perceived, but there is no doubt that the fastest decade on average was the 1960's.
Is this because of continuous inbreeding among thuroughbreds? Perhaps just a fluke of nature that will soon right itself? After all, if you drag the slider back to inception you can see that the trend has been undeniably upwards since the 1870's.