App Slowing down

I am working on Scrivener since three month. Very good writing experience. The task is a big Glossary, Amazon-related terms, actually with 700 articles and growing more. 82.000 words and with final target of million words.
Since one week I started to add photographs.I have no more than 100 picture and the final version can reach 10000.
I import photograph files as alias, to avoid long backup time
Starting with pictures created a strong after-before diference. The app is slowing down, each ten second the small rainbow ball rotate ten more seconds, and I have only to wait. The project file is +600 Mb, and I really hope that is just some simple adjustment of settings.

Thanks for helping, Leo