It takes me about as long to write something as to finish the footnotes and references. What I usually have to do is go and make a systematic list of every reference that is missing or incomplete, and then go back and fill those in one by one. I might need a book at the library, or an internet reference. This is very hard, tedious work to do, though not intellectually challenging. Precisely because it isn't intellectually engaging, it is tedious. It can be satisfying in the sense that it provides the sensation of completion. The article is not complete until every reference is correctly cited.
Why don't I do it as I go along? Because I am working very fast and intensely on the intellectual part of it. I might know that so-and-so talks about this in such a work, but I don't want to have to interrupt my writing, go to the library, get the book, and put down the page number. That would be very inefficient. I have finished most of the article I'm writing now, and which I started at the end of the semester. It has been laborious by highly pleasing.
I cannot start the phase of checking every reference until I have every sentence more or less written of the body of the text. Then I make a list. One trick I found is to go through the article and write the words "missing reference" every time there is one. Then I can search the document for those words and clean them up one by one.