How fast are baby zombies?
If the generic movement speed for normal zombies is 0.23, what is for baby zombies?
The "Baby speed boost" attribute modifier uses Operation:1 with value 0.5. This means that baby zombies have 150% the speed (or "50% more") compared to normal zombies.
As zombies normally have a speed of ~0.23, baby zombies have a speed of 0.23 * 1.5 = 0.345