If you're talking about a National Security Check, this thread here
https://www.enz.org/forum/showthread.php?t=51892 has quite a bit of information and guesswork (because detailed information is confidential - not like the procedures for processing visas). Three things I said over there...
The checks in this case rely on hearing from the foreign government what work the person did while in that country. Of course, this doesn't matter much to them, and they take their own sweet time, and the NZ authorities have no way of hurrying things up.
If you're talking about 'third party checks' for some aspect of a visa, the CO is going to be writing to anyone in a position to know that what you said about yourself is true. For instance, 'Yes, Mr X did attend our university between this date and that date, and was awarded a bachelor's degree in (whatever subject),' or, 'Yes, Mr X worked for our company as a project director responsible for Y between this date and that date.' The time this takes often can be stretched by the people not seeing anything particularly urgent about replying to the CO, who can't go on until each item has been satisfactorily ticked off.
As for your thinking "it's an excuse", no doubt you'll believe what you want. However, all of this is nothing personal to you, or any individual applicant. Everyone is in the same boat. Nobody has it in for the foreigner wanting a visa, or is trying to be unkind to them. The government has set procedures, and the officials have to follow them.