Efficient doesn't mean "fast". It means that it minimizes the ratio "energy_spent / goal".
So if we're going to talk about communication efficiency, we need to define what's the goal being optimized here.
If a manager wants to minimize the time spent dealing with one of the engineers and wants to see this engineer quit, then the "cursory explanation" can be considered "efficient".
If this same manager, instead, wants to maximizes the odds the engineer stays and improves her/his weak points, then the "comprehensive" explanation might be a lot more efficient, despite requiring the manager to spend more time at it.
