Slow loading

I’ve page with quizes. All quizes after call are loading very fast and work with them is OK. But on main page content of <div class=”vc_grid-container vc_clearfix wpb_content_element vc_basic_grid” …/> is loading very slow or even I cannot see them on user mode. Number of quizes is below 20.
What can be the reason of such slow loading?

https://ww.wp.xz.cn/plugins/wp-pro-quiz/