What's the practical impact of this speed boost on actual play though? One PC can catch up to a fleeing villain maybe? Stick and move with swashbuckler feature or Mobile feat? Kite with a bow? I'm not all that concerned about its impact on the game.
If it's that big a deal, have the villain create a special magical item to deal with this fast-mover:
Ball of Yarn
Wondrous Item, uncommon
As an action, you can throw this ball of yarn up to 60 feet. Any feline creature of any type (tabaxi, weretiger, rakshasa, lion, deadly house cats, etc.) that can see the ball of yarn must make a DC 13 Wisdom saving throw or become charmed. While charmed by the ball of yarn, on its turn the affected creature must spend all of its movement to move adjacent to the ball of yarn and, as an action, bat it away up to 30 feet in a random direction. The creature can repeat the saving throw at the end of each of its turns.