Subviews in dispatch_async appears after a long delay - ios

Question

I've used dispatch_async to put in background a xml document's parsing, I've putted information in an array and, with a for cycle, I would assign the content of every element to an UILabel (for now), the problem is that in the output console I can see the right content of every element but the uilabel are added only in the end after a long delay.
Code:
#define kBgQueue dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
dispatch_async(kBgQueue, ^{
NSData *xmlData = [[NSMutableData alloc] initWithContentsOfURL:url];
NSError *error;
GDataXMLDocument *doc = [[GDataXMLDocument alloc] initWithData:xmlData options:0 error:&error];
NSArray *areaCortina = [doc nodesForXPath:#"query" error:nil];
int i=0;
for (GDataXMLElement *element in areaCortina) {
NSLog(#"%#",[[element attributeForName:#"LiftName"] stringValue]); //data are shown correctly
UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, (30+10)*i, 200, 30)];// don't appear after log
[label setText:[[element attributeForName:#"name"] stringValue]];
[self.view performSelectorOnMainThread:#selector(addSubview:) withObject:label waitUntilDone:YES];
i++;
}
As you can see i've used performSelectorOnMainThread but nothig, the label doesn't appear once at once but are shown correctly only after a 10 or 15 seconds after the end of the block.
Ideas?
Thanks in advance
Ok edit 1
Thanks to the advice of Shimanski Artem I've now the following:
#define kBgQueue dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
...
dispatch_async(kBgQueue, ^{
NSData *xmlData = [[NSMutableData alloc] initWithContentsOfURL:url];
NSError *error;
GDataXMLDocument *doc = [[GDataXMLDocument alloc] initWithData:xmlData options:0 error:&error];
NSArray *areaCortina = [doc nodesForXPath:#"query" error:nil];
self.data = [[NSMutableArray alloc] init];
for (GDataXMLElement *element in areaCortina) {
[self.data addObject:[[element attributeForName:#"name"] stringValue]];
}
[[NSNotificationCenter defaultCenter] postNotificationName:#"name" object:self];
}
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(caricaItem) name:#"name" object:nil];
...
} //end method
-(void) caricaItem
{
int i=0;
for (NSString * string in self.dati) {
UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, (30+10)*i, 200, 30)];
[label setText:string];
[self.view addSubview:label];
i++;
}
}
I've put uilabels creation out of dispatch, in the caricaItem method where i've ready an array full of precious data, but same delay... and the same if in caricaItem i use a UITableView...
What is the right way to proceed?
Thanks