Algorithm for downloading images

I am working on a product application where user could sell/buy. This application is based on collection view. Collection view has collection cell where it displays product image thumbnail.

The following code gets products images from the server and it waits to download all images and then display them in the cells. The following code works but the user should wait 20-40 seconds to see products.

I highly suspect this implementation architecture is not good.

- (void)viewDidLoad {
    [super viewDidLoad];
     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
     dispatch_async(queue, ^(void) {
        [self loadFromURL];
     dispatch_async(dispatch_get_main_queue(), ^{
     });
  });
}

-(void)loadFromURL {
    NSURL *url = [NSURL URLWithString:@"http://myURL/productAll.php"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    operation.responseSerializer = [AFJSONResponseSerializer serializer];

   [operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) 
     {        
      pElements= (NSMutableArray *)responseObject;
      [collectionView reloadData];

     } 
     failure:^(AFHTTPRequestOperation *operation, NSError *error) 
     {        
       UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Error Retrieving Product" message:[error localizedDescription]delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil];
     [alertView show];
   }];    
  [operation start];
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return pElements.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    ProductCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"cellIdentifier" forIndexPath:indexPath];

    cell.backgroundColor=[UIColor whiteColor];
    NSString *arrayResult = [[pElements objectAtIndex:indexPath.row] objectForKey:@"image"];
    NSData *data = [[NSData alloc] initWithBase64EncodedString:arrayResult options:NSDataBase64DecodingIgnoreUnknownCharacters];
    cell.productImage.image = [[UIImage alloc] initWithData:data];
    cell.productImage.layer.cornerRadius = 10;
    cell.productImage.clipsToBounds = YES;
    return cell;
}