There can be delays based on the nature of the medical issue and the type of visa that it is.
Obviously INZ's MA are not specialists in every area of medicine and sometimes need to get opinions from DHB's and other professionals in the field in forming their opinions. This can obviously take time to do. They are also dealing with hundreds if not thousands of people at any one time.
There is also a crucial difference between a residence (and work to residence) applications and a temporary applications(Think student, visitor etc) because for a temporary they just have to ensure that an applicant in their proposed stay is not going to turn up to the hospital or specialist needing treatment.
Under a residence or work to residence assessment they need to do costings and check with dhb's etc in the area to make sure that your condition will not impact the overall publicly funded healthcare system. These measures are in place to protect the healthcare system from undue costs or use of already stretched resources because once you become a resident your ability to access the healthcare system changes. Most conditions get determined to be an Acceptable Standard of Health but you'll need to give them time to ensure that your conditions doesn't fall under those conditions that will pose a burden. You've also got to bear in mind they are doing this for all the applicants being referred to them and as with all things delays could be being caused by particularly difficult cases that they are trying to resolve.