Scanning large documents to PDF takes a long time

We are currently scanning a lot of historic documents through our C250. The scanner is configured to output a PDF onto a network drive. This works well with small files but can start to really struggle when we start to reach about 100-150 pages.

Typically a 50 page document will take about a minute from the time it stops scanning the batch to appear in the network drive. Yesterday I had a 700 page document take 4 1/2 hours. And this morning we had a 170 page document take 4 hours (before I deleted it).

The Job List shows the scanned documents sitting in the "transferring" status. This makes it sound like a network issue but I don't think it is. If it was it would be a uniform delay depending on the size of the document. The delay we are experiencing is almost exponential.

Any ideas?

I can only assume it's something to do with the printer memory or software it uses to create pdf's.