The bug

Apparently, horse breeding causes the child to inherit the final value of the parents' speed attributes, instead of the base value. As a result, parents with the speed potion effect produce offspring that are naturally faster than both. This process can be repeated indefinitely.

This is an active exploit that can be used in survival to obtain arbitrarily fast horses.

Note that this bug does not occur with jump boost due to MCPE-45823.

How to reproduce

1. Spawn two horses in an enclosed area
2. Tame them
3. /effect @e[type=horse] speed 500 10 true
4. Breed them
5. Feed the baby to adulthood, and ride it
→ It's ridiculously fast