Why touchesBegan has some delay in some specific area in UIView

I'm making an custom keyboard and I'm in a really weird situation.

I've noticed that when I catch the event touchesBegan at the rear left (about 20 pixels) of the UIView (inputView), I'll have some delay in here. Any action I do in touchesBegan will be perform slower than other area.

override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?)
{
self.keypop.hidden = false
}


override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {
{
self.keypop.hidden = true
}

I don't know why, or is this an error from iOS 9? I've been stuck on this trouble for a week.