What is the most time-consuming part of each project?

This question is impossible to answer, because a) there are so many different types of projects, and b) this varies as you become more senior!!

For example, an organisational re-design case will spend a lot of time in meetings with HR and Executives to define. refine, and re-refine the layers. On the other hand, in a Due Diligence case you might spend most of your time just setting up and gathering the input/assumptions required for the model.

I would also add that, at the risk of sounding obvious, as you become more senior, more and more of your time will be occupied by meetings - so the length it takes to e.g. conduct analyses will impact your day-to-day work less.