Core data - iPad - Fetch request on Table with > 300.000 Rows

Question

I have the following Coredata model:

Warengruppe contains the name of a specific product and WarengruppeVK is a disposal of a Warengruppe, it contains the selling price of a product and some other numeric values. PbsRow describes how many WarengruppeVK have been done in some timeframe, e.g. jahr=2012 and monat=2 tells us how many times a product has been sold to a specific customer in February 2012.

Now, I have a tableview and need to show how much revenue has been made for each specific product in some defined timeframe (user can define start and end dates). Additionally, I need to show how much revenue has been made in the same timeframe one year before.

So, this is how I'm doing it: First, I'm fetching all Warengruppe objects from Core data (12 products right now). Then, I'm using a for loop on my array of Warengruppe and calculate the sum of netto of each WarengruppeVK, using a predicate where PbsRow's jahr and monat are set to the user chosen timeframe. I'm defining the FetchRequest like this: NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"WarengruppeVK"];

NSExpression *keyPathExpressionNetto = [NSExpression expressionForKeyPath:@"netto"];
NSExpression *keyPathExpressionDB = [NSExpression expressionForKeyPath:@"db_basis"];

NSExpression *sumExpressionNetto = [NSExpression expressionForFunction:@"sum:" arguments:[NSArray arrayWithObject:keyPathExpressionNetto]];
NSExpression *sumExpressionDB = [NSExpression expressionForFunction:@"sum:" arguments:[NSArray arrayWithObject:keyPathExpressionDB]];

NSExpressionDescription *expressionDescriptionNetto = [[NSExpressionDescription alloc]init];
[expressionDescriptionNetto setName:@"sumnetto"];
[expressionDescriptionNetto setExpression:sumExpressionNetto];
[expressionDescriptionNetto setExpressionResultType:NSDecimalAttributeType];
NSExpressionDescription *expressionDescriptionDB = [[NSExpressionDescription alloc]init];
[expressionDescriptionDB setName:@"sumdb"];
[expressionDescriptionDB setExpression:sumExpressionDB];
[expressionDescriptionDB setExpressionResultType:NSDecimalAttributeType];

[request setPropertiesToFetch:[NSArray arrayWithObjects:expressionDescriptionNetto, expressionDescriptionDB, nil]];
[request setResultType:NSDictionaryResultType];

And here's the loop:

for (Warengruppe *wgruppe in warengruppen) {
    //predicateVJ defines the Timeframe for PbsRow    
    productPredicateVJ = [NSCompoundPredicate andPredicateWithSubpredicates:[NSArray arrayWithObjects:predicateVJ, [NSPredicate predicateWithFormat:@"warengruppe=%@",wgruppe],nil]];

    [request setPredicate:productPredicateVJ];

    NSArray *objectsVJ = [context executeFetchRequest:request error:&error];

    float nettoVJ = [[[objectsVJ valueForKey:@"sumnetto"] firstObject] floatValue];
    float dbVJ = [[[objectsVJ valueForKey:@"sumdb"] firstObject] floatValue];
    float dgVJ = dbVJ/nettoVJ*100;

    //Now doing the same for Timeframe minus 1 year (predicateJ)
    productPredicateJ = [NSCompoundPredicate andPredicateWithSubpredicates:[NSArray arrayWithObjects:predicateJ, [NSPredicate predicateWithFormat:@"warengruppe=%@",wgruppe],nil]];

    [request setPredicate:productPredicateJ];

    NSArray *objectsJ = [context executeFetchRequest:request error:&error];

    float nettoJ = [[[objectsJ valueForKey:@"sumnetto"] firstObject] floatValue];
    float dbJ = [[[objectsJ valueForKey:@"sumdb"] firstObject] floatValue];
    float dgJ = dbJ/nettoJ*100;

}

Now the thing is, it takes about 14 Seconds to fetch all the data. But if I remove the the second part (which begins with //doing the same for timeframe minus 1) it only takes 7 seconds.

So my question is, is there some correct way to split this up into 2 threads? I have already tried this with 2 dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ blocks, but for some reason it just takes the same amount of time (14 secs). Or is there some other way to improve this request? I can't use any fetch limits, since I need the absolute sum of these values. The big problem is the huge amount of data I'm using. It's an enterprise app which has to work offline, so there's no way for me to kinda calculate this stuff on some server and use http to pass it to the iPad.

WarengruppeVK has 328.666 rows right now :-/.

Thanks in advance.

EDIT: Here is a piece of the full screen on the iPad, after calculation was done:

VJ means last year and IST means the current timeframe

EDIT 2: predicateVJ and predicateJ are initialized like this (in this example, the chosen timeframe is current month):

NSPredicate *predicateVJ = [NSPredicate predicateWithFormat:@"(pbsrow.jahr=%d) AND (pbsrow.monat=%d)", year-1, month];
NSPredicate *predicateJ = [NSPredicate predicateWithFormat:@"(pbsrow.jahr=%d) AND (pbsrow.monat=%d)", year, month];

Then I'm starting the fetch request on a background thread:

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{

    NSArray *warengruppen = [[CoredataModelHolder sharedHolder] allWarenGruppen];
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
    [context setPersistentStoreCoordinator:[(AppDelegate*)[[UIApplication sharedApplication] delegate] persistentStoreCoordinator]];

    NSPredicate *productPredicateVJ;
    NSPredicate *productPredicateJ;

    for (Warengruppe *wgruppe in warengruppen) {
    //See 2nd snippet of my question.
    }
});