Try rewriting your question. I don't understand what you are asking.
Even the thread title, "Masters, PhD or Postdoc" is confusing.
The first two items, Master's and PhD, are academic degrees.
The third, postdoc, is an entry level faculty position, for which the minimum qualification, as the title suggests, is a PhD.

Apples and oranges. What is the question here?

Daniel --

Thank you for the clarification.

I suppose every person's experience will be different. I would expect that research for the PhD would be substantially harder than for a master's degree. In my field (mathematics), if a thesis is even required for a master's degree (a MS degree typically does not require one), it is not expected to contain new results, and is not expected to be published.

For my situation, a postdoc is expected to produce publishable results, at least if one's plan is to pursue an academic position at a research institution. The typical postdoc position has a light teaching load, but I have still found it to be a challenge to balance time between research and teaching duties. My supervising faculty member assures me that the amount of time available for research decreases as one moves from being a grad student to postdoc to assistant professor to associate professor to full professor. I suppose that one does not complete this progression unless they learn to become efficient and to master the time balance. So in one sense (time availability), research becomes more difficult over time.

On the other hand, I have found that some of the mechanics of research become a bit easier. One is already familiar with more of the literature in one's area, so new material is more quickly integrated, literature searches can be performed more quickly, etc. In other words, the more research one has already done, the more quickly or efficiently one can conduct new research. So ignoring the time availability, research becomes easier over time.

The above is my experience. Your mileage will vary. I'm not sure if this is what you had in mind regarding difficulty of research, but these are aspects that come to mind.

This topic caught my attention recently as I was reflecting on my own supervisor experience - I count myself lucky to have had a very hands on and supportive supervisor. Even now I try and keep in touch and their opinion is of great value to me.