I'm not sure why there was a need to "balance the difference in speed" when it comes to fighting unique NPCs. High-speed pets come with their own vulnerabilities, and now this will offset their only advantage.
NPCs that are of 'unique' quality now have an additional property:
Damage received from an enemy that has a higher Speed attribute is reduced by a multiplier dependent on the difference in speed (at maximum reduction, damage is reduced to 1/5 of normal value.)
Damage dealt to an enemy with a higher Speed is increased (up to a maximum of x5 damage).
This balances out difference in speed.
Will also work for pets.