Are the background images optimized?
ShortPixel Adaptive Images (SPAI) by default can optimize the images used as background only if they are included inline in the HTML of your website.
If the images are included in external CSS files, then SPAI won't be able to find and replace their URLs them unless you enable the Replace in CSS files option. To do this, go to Settings > ShortPixel AI > Areas and enable "Replace in CSS files", as you see in this image:
If the background images included in external CSS files are served from a different domain than yours, you should include such external domain(s) in the "Additional CSS domains box". Example: