Luckily, it really is only a few lines of thoroughbreds and mainly in the States (their habit of feeding their horses steroids may not help much either.) As for racehorse times, the record for our Derby stood from 1936 (Mahmoud) to 1995 (Lamtarra) and only one horse has beaten Mahmoud's time since (Galileo.)

The human times will almost certainly level out too (and drugs don't help there, either.)