The bonuses of artifacts (Amulet of Swiftness, Ring of the Traveler, and Cloak of Speed) are not taken into account;
The bonus of the hero's specialty, Sir Mullich (+2 to unit speed), is not taken into account;
The "home ground" bonus (+1 to unit speed) is also not taken into account;
Bonuses from creature specialties are taken into account (+1 to creature speed regardless of the hero's level), with the exception of specialties that provide a permanent bonus. These specialties are possessed by heroes of the "Conflux" castle and some campaign heroes.
But each castle has its own unit specialties that give +1 to speed and increase their boost.