Comment 8 for bug 2024887

TL;DR: Apparently the compression setting was set to the minimum value. Changing that fixed the problem.

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Investigation log:
  1. Pointing LD_LIBRARY_PATH to a restored backup of /usr/lib/x86_64-linux-gnu does not help.
  2. "strace -f" shows no external programs are called to handle compression/saving.
  3. Looking at the simple-scan sources, there must be a quality setting somewhere in the GUI? But it's not in Preferences, where I believe there used to be such an option?
  4. Found it: when the file format at the bottom of the save dialog is set to JPEG or PDF, a small "Compression" slider is shown next to it, which I had never noticed before (I always change file format by editing the file name + extension at the top). This was set to its minimum value.
  5. Moving the slider to the right fixes the issue for me.