UI get blocked when using NSURLSession

I'm working on a project which requires a login form, using a webservice for authentication. I have no issue connecting to the server but it seems that NSURLSession blocks my user interface and I really dont know why after a lot of debugging.

For simplicity, here's my code in short:

NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:@"example.com/service"]];

//1
    //_sessionLogin = [NSURLSession sessionWithConfiguration:sessionConfigurationLogin delegate:self delegateQueue:nil];
//2 //Whether I use 1 or 2, it acts the same way

    _sessionLogin = [NSURLSession sharedSession];

    NSURLSessionDataTask *sessionDataTaskLogin = [_sessionLogin dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error)
    {
        if(!error)
        {
            NSLog(@"loginWithSuccess");
            UIAlertView *alertError = [[UIAlertView alloc] initWithTitle:@"Login ok" message:@"ok" delegate:self cancelButtonTitle:@"Close" otherButtonTitles: nil];
            [alertError show];

    }
}];

//Begin login request
[sessionDataTaskLogin resume];

_sessionLogin is a NSURLSession

As the connection to my server is fast, the NSLog(@"loginWithSuccess") appears almost right after I pressed the login button but it takes me a while (a long long while) about more than 10 seconds that the UIAlertView is showed. And I can't interact with the UI too.

Thanks in advance for every solution.

Your completion block is not running on the main thread. Since UI updates have to happen on the main thread, you should dispatch the alert view to the main queue, and you'll see it immediately.

NSURLSessionDataTask *sessionDataTaskLogin = [_sessionLogin dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    if (!error) {
        NSLog(@"loginWithSuccess");
        dispatch_async(dispatch_get_main_queue(), ^{
            UIAlertView *alertError = [[UIAlertView alloc] initWithTitle:@"Login ok" message:@"ok" delegate:self cancelButtonTitle:@"Close" otherButtonTitles: nil];
            [alertError show];
        });
    }
}];