It might be small, but the dragons can't reach the other side diagonally.

under standard conditions! But there are ways to increase speed or reduce distance.
1) Boots that give +1 speed.
2) Tactics that add +1 cell.
3) The "native lands" skill from logistics usually increases speed by +4 during the siege of an enemy castle, but for Elves and Haven, the bonus also works in a normal field battle if the battle takes place on "grass" terrain (I personally have never leveled up a hero this way).
There is also a speed rune (increases monster movement by 100%) and a barbarian warcry that gives +1 speed to monsters... but they are only available to priests and barbarians, respectively!