i agree, scribus should manage the images as thumbnails...
but you, too, should manage the images at the correct size :-)
it's unlikely that the 90 images you're using in 12 pages can be 1.7 GB big if you resize them correctly to match the resolution you need for printing.
what you can do now: duplicate the directory containing all your images, rename the duplicate in a way that it states that it contains the original images and resize all the images linked by scribus (you can use a software like phatch to do automatically do it).
btw, as far as i know there were a patch to let scribus use some cache... but it's possible that it has only been applied to the development version...
ciao
a.l.e