Generally speaking, the larger the horse, the longer it takes to pulse down. In cool climates, this is not as critical as hot, humid climates.
You know how some horses work a little bit, and you can see a lot of their veins pop up? That is a good indicator of how the horse cools itself, and therefore lowers its pulse.
Different breeds also have different types of muscles. QH have "fast twitch" muscle type, good for bursts of speed, but not long distances, without a lot of conditioning. Those muscles are usually bigger, and hold heat.
Arabians have "slow twitch" muscles, which are good for long distances, and the muscles are smaller and stringier, so they do not hold as much heat.
Endurance as a sport is really all about the human being able to READ the horse, and adjust the goals on the fly for the benefit of the animal. You may start out wanting to WIN, but adjust because the horse is hot, thirsty, hungry.... You may start out saying, " I am staying at the back of the pack, going slow", and the horse just keeps trucking along, and ends up first!!!!