There's a stickied discussion in the General Forums on the difference between DPI and PPI, its worth a look, based on some of the discussion here. But to answer your question, it really depends on what software you are using. Sounds like Photoshop or GIMP, both with lots of layers and pixel area are problematically slow. That's why I use a vector app with robust image handling capability, Xara Xtreme, and that I create most of my maps with the intention of printing - as I run Gamer Printshop and specialize in printing large format maps. If you're using an image editor and using lots of layers, performance will go down, its how the application manages memory.

Good luck in your issue, and welcome to the Guild.

GP