A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd?

When the milk is made more alkaline by adding a base to it, it is basically done to prevent it from to convert into curd faster.

As this milk is slightly more alkaline than other, therefore, acid produced to set into curd will be neutralized by baking soda added by milkman. Hence, this milk takes a longer time to set as curd.