Rendimiento de la suma de datos básicos

I have some theoretical question to ask about Core Data and sum función.

I try to sum values from Datos principales table with three ways.

1. fetch all and use expression to sum it up :

   NSArray * array1 = [self getAll:self.managedObjectContext];
   int sum = [[array1 valueForKeyPath:@"@sum.sum"] intValue];
   

2. fetch all and use for loop:

   int sum2 = 0;
   NSArray * array2 = [self getAll:self.managedObjectContext];
   
   for (Test * t in array2) {
       sum2 = sum2 + [t.sum intValue];
   }
   

3. let Core Data sum it.

   NSArray * array = [self getAllGroupe:self.managedObjectContext];
   NSDictionary * i = [array objectAtIndex:0];
   id j = [i objectForKey:@"sum"];
   
   (NSArray *)getAllGroupe:(NSManagedObjectContext*)Context{
   
       NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
       NSEntityDescription *entity = [NSEntityDescription entityForName:@"Test"  
                                      inManagedObjectContext:Context];
   
       NSExpressionDescription* ex = [[NSExpressionDescription alloc] init];
       [ex setExpression:[NSExpression expressionWithFormat:@"@sum.sum"]];
       [ex setExpressionResultType:NSDecimalAttributeType];
       [ex setName:@"sum"];
   
   
       [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:ex, nil]];
       [fetchRequest setResultType:NSDictionaryResultType ];
   
   
       NSError *error;
       [fetchRequest setEntity:entity];
       NSArray *fetchedObjects = [Context executeFetchRequest:fetchRequest error:&error];
   
   return fetchedObjects;
   }
   

surprisingly the

1. way was the slowest (for 1.000.000 data --> 19.2 s), el
2. way was faster (for 1.000.000 data --> 3.54 s) y el
3. way was the fastest (for 1.000.000 data --> 0.3 s)

¿Por qué es esto?

If I understand right even core data need to go through all 1.000.000 datas and sum it. Is this because use more cores if there are available?