It seems calling presentViewController:animated:completion from within tableView:didSelectRowAtIndexPath: is problematic. It’s difficult to find anything that stands out when using the Time Profiler in Instruments, also. Sometimes my modal view comes up in less than a second and other times it takes 4s or even 9s.
I think it’s related to the underlying UIPresentationController and layout, which is one of the few things I see when selecting the region of time between tapping on a row and seeing the modal presentation in the Time Profiler.
A Radar exists describing this issue: http://openradar.appspot.com/19563577
The workaround is simple but unsatisfying: delay the presentation slightly to avoid whatever contentious behavior is causing the slowdown.
dispatch_async(dispatch_get_main_queue(), ^{
[self presentViewController:nav animated:YES completion:nil];
});