BWW,
You're right that opening a very large PDF for each of your pages and seeking that large PDF for a single page is very inefficient, and your solution is better (and you can split the PDF using Workflow and the PDF tools, so there's that).
In these sorts of situations, for example if splitting the PDF isn't an option, we generally recommend using the
Alambic API (aka PDF API) in order to "stamp" one PDF onto the other, one page after another. This sort of implementation is scripted, and since it loads the PDFs at the start and only once, it's very fast and efficient.