Why do some applications take a long time and some get decided really quickly?

Apart from the fact that different types of applications are more or less complex, residence applications have lots of things to be checked so they take a lot more time for example, every application type is different and requires different things. You can check the processing times on INZ's website, it is clear there that processing times for visa types are all different, but what about for the same visa type?
A visitor visa from one person will be very different to a visitor visa from another person and so the application time will also be different. The assessing officer may be a fast worker, or just have a good or bad feeling about the application, or be really really busy. All of these things can affect the processing speed. Just because you are from the same country and put in the same supporting documentation as your friend does not mean that the processing times will be the same. Each application must be tailored to the applicant completely.
One thing that we do to make the processing speed the best possible is to have everything that the assessing officer needs all from the very start. When your application gets submitted, it is checked briefly to see if you have all the necessary documents. Once this is OK it is is put in a pile to be assigned to a case officer. Just because the application has all the necessary documents for an application, doesn't mean the case officer will be satisfied. When they need to ask for more information, this is one time that the processing slows right down. They have to put down your file, ask for the information, wait until it arrives, and then find the time to get back to your application. When the case officer is waiting for your new information they will of course be working on other peioples' applications so yours goes down the priority list. When the case officer gets back to your application they need to get back up to speed and start working on yours again. This all takes time.
Once an application has been submitted there is not much you can do to make INZ hurry up, all you can do it put in a complete and strong application from the start. INZ are the NZ Government, it is obviously not a good idea to ask them to hurry up with your application all the time. This is why we always do out best to put in the best application we can every time.