From the information you provided, I think of a similar situation I encountered in the past. So, let me say my guess based on my experience.
1. If your image is very large (large file size and/or large resolution, e.g. 5000x3000), Chrome will have a noticeable delay to display the image.
2. When you upload images to a CDN provider, by default, CDN providers will compress the picture enormously(hugely) to speed up network transmission. After that compression, the picture has become very small and the resolution has been reduced. Thus Chrome will display it quickly.
Could you compress your background image to a smaller size, and test it again?
Another suggestion, remove all other resources, just only test image load speed, like this:
<img src="image.jpg"> <!-- test jpg format -->
<!-- Or test webp format <img src="image.webp"> -->