A long agenda also does not necessarily mean a long meeting. As in weight and volume, content is the main determining factor. First on the business part of the agenda is the consent portion. Usually this is the more mundane business. It also is basically administrative in nature and does not require discussion or debate on issues or policy.
New business is usually the most exciting part, as is most anything new to anybody.
New does not always mean new either. Quite often the issue has been worked on by a committee or committees for several weeks. It is merely new, and relatively exciting, to the council as a whole. Old business is quite often more controversial than other business. Usually the issue has been continuing over a period of time, or the council has had questions or concerns and has delayed acting impetuously on an item.