Red flag - long interviewing process In December I applied for a post doctoral position. After 4 months I had my job interview and now one month later they are ready to make a decision.

Is a long interviewing process a red flag? I am concerned that this shows that the department has a bad organizational structure and decisions are slow to be made.
That is pretty normal. That does not mean it is good, but it is not worse than your average university. Two things play a role here: 

First, universities are just large organizations with all the bureaucracy and inefficiency that comes with that. 

Second, universities are often self organizing, that is, the professors take a large part of the management duties. That is a good thing, in the sense that decisions are made by people who know what they are talking about. But it also means that a large part of those tasks are performed by people who are not primarily managers and have competing priorities (research, teaching).