Normally we used to do something like this:

Map<Id,Contact> mapContact =  new Map<Id,Contact>();
public static void addListToMap(List<Contact> conList){
	for(Contact con:conList){
		mapContact.put(con.id,con);
	}
}

Here the processing time will be more(CPU execution time will be more), because we are iterating over a for loop to construct a map.

The best way to construct the map where we can avoid iteration like this:

public static void addListToMap(List<Contact> conList){
	Map<Id,Contact> mapContact =  new Map<Id,Contact>(conList);
}

Here the processing time will be very less.