I've actually got my Vaniya map spread across seven or eight PSD files. I found that having it all in one was getting far too slow to process and even now, spread over so many different files some of them (like the effects files that adds paper texture, aging, folds etc.) are pretty big and take a long time to load.
I'm finding it much easier though since I split it up into processes. I have a base map file which is my black and white land mass which I work on for rivers, a file for the coastlines, land edge and basic green colour. A file for the different country colours and borders. A file for the map features such as mountains, hills and trees. One for the outside border. One for the cartouche and rose. One for the text and one for the effects. Phew.

It might not work for everyone but I sure find it helps keep my PC from complaining too much.