I found this post on Elementor support :
Elementor Pro Tip of the day: If you use a CDN and notice that your Elementor section background images are still calling from the WP Media library, and not the CDN address then use this trick. This fixes your YSlow score. Upload the image to a post and copy the CDN address. Also, use the inspect element tool to find your background class CSS for that section. Then add the image in via the additional CSS area instead of the Elementor Style tab. This can help get your YSlow up to the 100.