Resolution size of an image is the most important thing you need to watch. An image can be 3400 px wide, but if it’s resolution is 72 dpi, it will load fairly quickly. If the resolution is 300 dpi, then it will just come to a crawl.
Example below has a 300 dpi and a 72 dpi. Click on each image and you’ll see the difference in how fast they load. The 300 dpi will slowly come in much slower.