FreeCol:Colopedia.concepts.efficiency.description/en

The efficiency of your colonies depends on their support for the idea of independence. If 50 percent of the colonists favour the rebel cause, the colony is granted a +1 production bonus. As soon as 100 percent of the colonists become rebels, the production bonus increases to +2. On the other hand, if the number of royalists exceeds the bad government limit, the colony incurs a production penalty of -1. Should the number of royalists exceed the very bad government limit, the production penalty is increased to -2. These two limits depend on the difficulty level and can be checked in the difficulty level report. In order to increase the efficiency of a colony, you need to reduce its population, or produce more liberty bells.