How to control Save as PDF behaviour

Changed a template for a book and now it takes FOREVER to create a PDF. I checked the PDF settings - no bookmarks are created apart from the main elements, no tagged PDF is created. Still, I can see that FM is creating an almost 2 GByte tps file for a book that is just over 5 MBytes, and taking an enormous amount of time to do it. Another version of the same book was done in seconds.

What am I missing here ? What should I look for in what part of the document settings ?

Thanks for any useful pointers