If several bags of cement are added to the cart being pulled by a horse, the overall weight of the cart increases. As a result, the horse would need to exert more force to overcome the increased mass of the cart.
Assuming the horse can only generate a limited amount of pulling force, the speed of the cart would likely decrease because the additional weight would require more effort to move at the same speed. If the horse's pulling force remains constant, it may reach a lower steady speed with the added load or take longer to accelerate to its new speed.
In summary, adding weight to the cart would generally reduce its speed, assuming all other conditions remain the same.