more important is the amount of data in Qlik. Not your final document. Nprinting must filter throug all the data.
Besides Qlik are there many other things. How busy is your Qlik server and your nprinting server.
You can creatie a Qlik document with only the data and objects for nprinting. This will speed up your process.
I had similar issues. There are a few things to check.
1. Data - This is before the App showed 1 month of data now it shows 12, or before it was aggregated now its showing individual rows
2. Increase of Sheets - Maybe before you only were populating 4 Sheets now there's 8 sheets
3. Increase of data on each sheet - Maybe before you had 1 chart now you have 3 charts in a Excel sheet
4. Aggr done in Excel - This is where you are doing a resource heavy Template in Excel (i.e sums etc)
5. Bandwidth - If you find everything else to go okay remember these need to be emailed so if your bandwidth decreases this will slow the distribution
The above affected me when i was developing so i thought i would share.