tableview 加载图片卡顿问题

- (void)loadImageFromURL:(NSURL*)url {

    NSURLRequest* request = [NSURLRequest requestWithURL:url
                                             cachePolicy:NSURLRequestUseProtocolCachePolicy
                                         timeoutInterval:60.0];
    connection = [[NSURLConnection alloc]
                  initWithRequest:request delegate:self];
    //TODO error handling, what if connection is nil?
}

- (void)connection:(NSURLConnection *)theConnection
    didReceiveData:(NSData *)incrementalData {
    if (data==nil) {
        data =
        [[NSMutableData alloc] initWithCapacity:2048];
    }
    [data appendData:incrementalData];
}

- (void)connectionDidFinishLoading:(NSURLConnection*)theConnection {
    
    connection=nil;
    
    if ([[self subviews] count]>0) {
        [[[self subviews] objectAtIndex:0] removeFromSuperview];
    }
    
    UIImageView* imageView = [[UIImageView alloc] initWithImage:[UIImage imageWithData:data]];
    
    imageView.contentMode = UIViewContentModeScaleAspectFit;
    imageView.autoresizingMask = ( UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight );
    
    [self addSubview:imageView];
    imageView.frame = self.bounds;
    [imageView setNeedsLayout];
    [self setNeedsLayout];
    data=nil;
}

- (UIImage*) image {
    UIImageView* iv = [[self subviews] objectAtIndex:0];
    return [iv image];
}

- (void)dealloc {
    [connection cancel];
}

@end

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

    static NSString *CellIdentifier = @"ImageCell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    
    if (cell == nil) {
        cell = [[UITableViewCell alloc]initWithFrame:CGRectZero reuseIdentifier:CellIdentifier];
    } else {
        AsyncImageView* oldImage = (AsyncImageView*)
        [cell.contentView viewWithTag:999];
        [oldImage removeFromSuperview];
    }
    AsyncImageView* asyncImage = [[AsyncImageView alloc]initWithFrame:CGRectMake(0, 0, 320, 200)];
    asyncImage.tag = 999;
    NSString *urlString = [NSString stringWithFormat:@"http://www.sd.xinhuanet.com/news/yule/2016-01/26/1117895557_14537752611851n.png"];
    NSURL* url = [NSURL URLWithString:urlString];
    [asyncImage loadImageFromURL:url];
    
    [cell.contentView addSubview:asyncImage];
    
    return cell;
}