Hi,
... I thought its a lazy load image problem.
Lazy Load is a jQuery plugin written in JavaScript. It delays loading of images in long web pages. Images outside of viewport (visible part of web page) wont be loaded before user scrolls to them. This is opposite of image preloading.
...But we don't use this JS on our template any more.
Delay with images it's normal, because the more important for browser is the rest of content not "BIG" images. It may be also connected with images file weight. If they have more then 100kb each it will take a while to load.