Images disappear for a few seconds when switching tabs. Who faced it?

Question

PHP

Greetings friends! The situation is this: there are several images on the site, some are menu items, others are ordinary images on the main page. For the first time I encountered such a situation and I don’t know where to start digging ... The fact is that I switch the tab, look, for example, for information on another site, and when I switch back, there are no images! But this does not last long, at first only the frames around the images (border) are visible, then all the images begin to appear in a chaotic manner. In the slowest and oldest browser, it takes about three seconds, in the new one. I didn’t set a caching ban, and I haven’t seen this on any site. I use the boilerplate template, but I think it has nothing to do with it. The site itself is dynamic, in PHP, links to images are loaded from the database, but even if I refresh the page this does not happen, It is when switching tabs and after a while. Please, if anyone knows, explain to me what's going on?