NSURLSession加载速度很快，但UI更改需要很长时间

I'm making an app that uses NSURLSession to get data from a "php backend". I used NSLog to check how much time it takes to get the data, and the data comes in almost instantly. However, once I hook it up to change the text of a UILabel, it takes more than a few seconds for the text to change. Isn't NSURLSession supposed to relegate fetching to background threads so UI interactions are faster? Also, is there any way to speed this up? Here's my viewController code:-

    //
//  hoursViewController.m
//  NSS IITD
//
//  Created by Robin Malhotra on 22/05/14.
//  Copyright (c) 2014 Robin's code kitchen. All rights reserved.
//

#import "hoursViewController.h"

@interface hoursViewController ()
@property (strong, nonatomic) IBOutlet UILabel *hoursLabel;
@property (strong,nonatomic) NSArray *dataArray;
@end


@implementation hoursViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}
- (IBAction)checkHours:(id)sender
{
    NSString *URL=@"http://www.nssiitd.in/nsshours/teststudent.php?entry_no=2012TT10951";
    NSURLSession *session = [NSURLSession sharedSession];
    [[session dataTaskWithURL:[NSURL URLWithString:URL]
            completionHandler:^(NSData *data,
                                NSURLResponse *response,
                                NSError *error){
                NSString *dataString=[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
                NSLog(@"%@",dataString);
                self.dataArray=[dataString componentsSeparatedByString:@";"];

                for (NSString *string in self.dataArray) {

                    NSLog(@"%@",string);

                }

                NSLog(@"lasdt valiuse is %@",[self.dataArray objectAtIndex:2]);
                UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"Hours" message:[self.dataArray objectAtIndex:4] delegate:self cancelButtonTitle:@"okay" otherButtonTitles: nil];
                [alert show];
                [self.hoursLabel setText:[self.dataArray objectAtIndex:4]];

            }] resume];


}

- (void)viewDidLoad
{
    [super viewDidLoad];




    // Do any additional setup after loading the view.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

@end

And "Here's" the entire project on "gitHub".

Try dispatching the UI code in the main thread:

[[session dataTaskWithURL:[NSURL URLWithString:URL]
        completionHandler:^(NSData *data,
                            NSURLResponse *response,
                            NSError *error){
            NSString *dataString=[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
            NSLog(@"%@",dataString);
            self.dataArray=[dataString componentsSeparatedByString:@";"];

            for (NSString *string in self.dataArray) {

                NSLog(@"%@",string);

            }

            NSLog(@"lasdt valiuse is %@",[self.dataArray objectAtIndex:2]);

            dispatch_async(dispatch_get_main_queue(), ^{
                UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"Hours" message:[self.dataArray objectAtIndex:4] delegate:self cancelButtonTitle:@"okay" otherButtonTitles: nil];
                [alert show];
                [self.hoursLabel setText:[self.dataArray objectAtIndex:4]];
            });

        }] resume];