I think part of the reason there's no monster tag for speed is because the monsters usually only take a "turn" when the players miss a roll or look to the GM to continue the conversation. Dungeon World doesn't have initiative like other games, so saying a monster is fast doesn't mean it'll be able to interrupt a player's action or go before the PCs as in other games.
That being said, giving a monster a quick, nimble, or ponderous tag would give the GM an indication of how to describe it's actions in the fiction of the game. If I saw a monster with the ponderous tag, I'd probably have it do less when describing it's action/reaction. A ponderous beast might slowly turn towards the PCs and raise it's weapon, rather than being able to strike and do damage right away. A nimble creature might duck under a shield, make a few quick jabs, and then hop away, forcing the PC to close with it again.
So, I see no reason why you can't come up with new speed-related tags if it'll help you remember how your NPCs are meant to act during play. But you could also include a note about their speed (or lack thereof) in the creature's description, or in it's list of moves. The ponderous creature could have "Act/react slowly, but forcefully" as a move, just like the nimble creature could have "Act/react with unnatural speed".
If you like to do things "by the book" (as I do), you could modify one of the questions from the "Making Monsters" section (p 227). For example, "It isn't dangerous because of the wounds it inflicts, but for it's speed and agility: nimble, reduce it's damage die by one size, write a move about it's speed or agility." Or "It's slow witted or slow moving or both: ponderous, +2 damage or +2 HP or both (your call)."