pdftoppm and Ghostscript (invoked via Imagemagick) re-rasterize full pages to generate their output. That's why it was slow. Even worse with a Q16 build of Imagemagick. Better to extract the scanned page images directly with pdfimages or mutool.

Followup: pdfimages is 13x faster than pdftoppm

This. Not only is it faster, the images are likely to be of better quality. If you rasterize the pages then the images will be scaled, unless you get very lucky.