The Kentucky Derby horse race has been run since 1875.
Horses were getting faster for a while, but speeds appear to have leveled off.
Secretariat's time in 1973 is still the record.
A scatterplot of Time by year reveals that the length of the race changed in 1886. For that reason, Speed is a better variable to examine.
It may be interesting to compare these races to the corresponding ones in the same years for the Belmont Stakes.