
Undergrad papers are much easier if you are writing within an area you've studied because you can simply regurgitate what you already know.
As for Postgrad, you'll need to do more research because most of them demand use of theories and concepts that you first need to have already mastered before doing your research.
This is the reason it is quite difficult to write a postgrad paper in an area you are not familiar with.
Also, most postgrad papers demand use of more sources which again means that there is more legwork on it unlike undergrad where in most cases 1source/page is mostly more than sufficient.
Under pressure, I can even write 5 pages/hour if it is an undergrad paper (good quality) but I cannot say the same of postgrad.