Removing blur effect iOS 8 takes a few seconds - ios8

Question

I implemented UIBlurEffect in my view controller when my UIAlertController appears via self.view addSubView method. However, in the action code where I had dismissed the alert controller, it takes a couple of seconds of lag before the blur effect is removed. Is there any reason for the lag?
// Create effect
UIBlurEffect *blurEffect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleLight];
// Add effect to an effect view
UIVisualEffectView *visualEffectView = [[UIVisualEffectView alloc] initWithEffect:blurEffect];
visualEffectView.frame = self.view.frame;
UIAlertController *alert = [UIAlertController alertControllerWithTitle:#"Input Required" message:#"You are using UIAlertController" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *cancel = nil;
if(self.m_txtStaffID.text.length == 0)
{
// Add the effect!
[self.view addSubview:visualEffectView];
cancel = [UIAlertAction actionWithTitle:#"Dismiss" style:UIAlertActionStyleCancel handler:^(UIAlertAction * action)
{
[self.m_txtStaffID becomeFirstResponder];
[visualEffectView removeFromSuperview]; // this takes a full 1-2 seconds to take effect
[alert dismissViewControllerAnimated:YES completion:nil]; // but this is almost immediately!
}];
[alert addAction:cancel];
alert.message = #"Enter your staff ID and try again";
[self presentViewController:alert animated:YES completion:nil];
return;
}