How are you doing the export? Are you using on demand or running a task?
Running tasks from Nprinting designer seems to run faster than On-Demand. I see 3 to 1 performance improvement this way.
Are you using filters or current selections? Filters bring the whole data set across to Nprinting whereas current selections only bring across the reduced selected data set.
I am exporting 60 images to 30 slides and it takes 35 seconds in a task but takes 2 or more minutes using On Demand.
If you have more information to share, I might have a few other ideas
Hope that gets you started.