I've recently started a big project, scanning a lot of magazine and reducing them to PDF files.
So far Gimp have served me well, I've succeded in automating a lot of my workflow, which can be basically summarized as:
* open image
* set brightness level
* export as PNG
The files I'm working on are ~20mb RGB 24bit 2500x3400 pixels PNG files.
There is a lot I can automate if I pay attention to the scanning process and create homogeneous source files.
I've started working on this project on Gimp 2.8.x and moved to 2.10 recently, using the same scripts. I was able to directly compare both releases on the very same workflow and noticed that 2.10 is much faster and manages memory better; that translates to being able to open more files at once and closing them faster.
I'm using PDB to automate as much as possible. Learning the API is not an easy task because the documentation is scarce and in my opinion the Python API could be improved/extended. Still, an impressive feature.