On smartphone some time website open too slow Why ? do you guess the reason .
Ofcourse if none other than the Images which are big in size and loading to slow in mobile and desktop browser. Always new designer use the big size of images into website and face the speed issue. website ranking will goo down if open too slow.
Use the photoshop "Save as Web" menu to save images for website
Always use the DPI low so that size will be low.
Use the image size same as you need , dont use big images to show them thumbnail
Use PNG images if need to show background transparent so that size will be low but recommended extension is always JPG .