I finished a project recently that took up around 10Gb of RAM on my machine, for 164 pages with heavy graphic usage (https://www.drivethrurpg.com/product/353349/SINS--MANIFEST-DESTINY - you can view a preview to see how it's laid out)
My machine: Windows 10 64bit 32Gb RAM available. Just loading Publisher uses 0.7Gb.
So I did a test creating a document with the same 100-page filler text (I copy/pasted the afpub file from the same master, only changing colour space as appropriate), and the same 10 linked images (+2 images on the master page that are copied to each page with a mirror transformation, all of which are in the appropriate colour space but in different file formats);
CMYK8 AfPhoto 1.3Gb
CMYK8 PSD 1.5Gb
CMYK8 TIFF 1.1Gb
RGB8 AfPhoto 1.6Gb
RGB8 PNG 1.1Gb
RGB8 PSD 2.0Gb
RGB8 TIFF 1.1Gb
These seem quite large RAM usage, and it is with a clean install of Publisher 1.9.2.1035.
Definitely something that needs looking into. Perhaps by working with low-res placeholders of the images rather than loading each image into RAM (which is what I suspect is going on so you can edit them directly).
Adding an option to the 'linked' files to allow low-res placeholders until export would be great, as well as having an option to have the links be relative to the afpub file (assuming it's all nested, of course)