Firstly you should check your website performance. You can use Google Page Insights. It presents a report related to your website. You can examine this error attentively. Then you start to solve this problem. Generally, image size or format causes this slow loading time. You should optimize your image, maybe use CDN. image4io can solve these problems. Because image4io is an image optimizer, using CDN, storage management. After you integrate to image4io, your website speed up seriously. Try it!