CoreData Sum Performance

I have a theoretical question to ask about Core Data and the sum function.

I am trying to summarize values ​​from a Core Data table in three ways.

• extract everything and use the expression to summarize:

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

• extract everything and use for loop:

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

• let Core Data summarize 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 path was the slowest ( for 1.000.000 data --> 19.2 s)
• the path was faster ( for 1.000.000 data --> 3.54 s) and
• the path was the fastest ( for 1.000.000 data --> 0.3 s)

Why is this?

If I understand correctly that even the main data must go through all the 1.000.000data and summarize them. Is it because using more cores if available?