I have published several photo books with text with some 150+ pages in total. Scribus 1.4.4 and 1.4.5 on an 8 year old iMac 8.1 (24", 3.1 Ghz, 6 GB RAM, SSD Drive, OS X Mavericks through El Capitan). Most of the images are 16bit TIFF, 300 dpi and full page size. Loading the document takes a while, because the images must be loaded from an external FireWire 800 drive, but once loaded, Scribus is running smooth without any hickups.
Generating a PDF/X-3 also takes a while, because the images in the document have to be resampled - most of them are considerably larger than page size and I am too lazy to scale them before inserting them into the pages.
End of last year I have published a 50 page reference book with text and images on 350 g/m² Chromosulfat carton. Page size: 46 x 32 cm = 18,1 x 12,6 inch. Width when opened: 100 cm = 39,4 inches = 3,28 feet.
Most of the images I used were scanned images from a large format camera at 13.020 x 8.680 pixels, 16bit, color profile ECI RGB 2. The result is awesome:
https://beefoto.wordpress.com/2016/03/04/muster-buch/Just to prove that even if you have an old but 'clean' machine with below average RAM and an old CPU you can rely on Scribus.

