I do not know if it would be efficient for me to include the data type Map as a variable in Entity.If you mean std::map, then it is most probably not efficient to use it as a copy, and a reference ('&') is more effective.
Either use "const std::map<T1,T2> &arg" if you only want to use it to look up values, or use "std::map<T1,T2> &arg" if you need to change the map.
If this map is part of the Entity class, then you should define it as a member of the Entity class instead, and you will have immediate access to it that way (no need to access it as an argument).
A word about efficiency: the standard procedure is to first make the general design, then benchmark to find the bottlenecks, and last thing to optimize. Otherwise, there is a high risk that you spend effort on optimizing things that have no real effect on the application, or even worse, create bad design.