Checking the pasteboard's string by if string == UIPasteboard.general.string takes a couple of seconds if you have long sentence in the pasteboard. The user sees the keypad is frozen while this check. My solution is to check if the length of new characters is longer than 1. If it is longer than 1, the string is from the pasteboard.
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool { if string.characters.count > 1{ //User did copy & paste }else{ //User did input by keypad } return true }