if the images are already optimzied, you can try to avoid that scribus recalculates the images by compressing them when exporting to pdf.
if you want more help, i think that you'll need to share your document, collected for output and with everything (images and fonts included) zipped.
ah, and in this case, the scribus version might matter: which version of scribus are you using?
ciao
a.l.e