I had a few minutes to write this morning. I managed to get one footnote written. It wasn't even a long footnote with an interesting but tangential argument. Just a simple: "On this topic, see A, and also B and C."
But if you start to write a footnote, you'll want to double-check the page numbers in your sources. When you check the page numbers, you'll want to check the footnotes, too. While you're checking the footnotes, you might notice that your source cites some primary material you haven't seen before. When you try to find a digital copy of the promising new material, you might stumble onto a whole digitalization project with 3000+ sixteenth-century books (so far) that you have never browsed before.
I've only made it through the first 600 items from the ULB Sachsen-Anhalt/Universität Halle's list of sixteenth-century digital editions, but there were a few high-priority items, including two 1516 editions of the Extract of Various Prophecies. I only got one footnote written, but it was still a morning well spent.