dismissViewControllerAnimated有延迟

dismissViewControllerAnimated:completion:在应用中运行正常，就是不知道为什么出现了几秒钟的延迟：
[api loginWithUsername:[dict objectForKey:@"username"] andPassword:[dict objectForKey:@"password"] andSuccessBlock:^(id json) {
    NSLog(@"DONE... %@", [json objectForKey:@"status"]);
    NSString *status = [json objectForKey:@"status"];
    if([status isEqualToString:@"ok"]){
        app.user = [json objectForKey:@"data"];
        [self dismissViewControllerAnimated:YES completion:nil];
    }else{
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" message:@"could not log you in" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil];
        [alert show];
    }
}];

在控制台可以看见DONE... ok。，意味着回调block执行正常，但是接下来需要三四秒钟延迟视图才能最终被dismiss。

怎么解决这个延迟问题？