Journal3 blog posts are a class of pages that NitroPack does not optimize by default. Fortunately, telling NitroPack to optimize these blog posts is very easy:
Go to the NitroPack extension: Admin > System > NitroPack.io > (Your Site)
Find the section called Page Types > Custom Page Types
Click on Include Page Type
Add a page type for the route journal3/blog
You can also configure another page type for journal3/blog_posts. This should optimize your blog listing page as well.