Love it, feels like browsing the web about 20 years ago. (Or a few bad websites now…)
So this is an example of Cumulative Layout Shift (CLS)[0]. It is a metric (coined and named by Google) which indicates how much “movement” there is during the initial render of a page, caused by reflowing the layout. Ideally you have very little or none of it for better UX. I believe it can have an impact on organic search ranking and potentially ad rank.
The most common cause is images included without width and height attributes, therefore the page has to reflow after the image has been downloaded.
Other things that can cause it are multiple css files overriding each other and loading slowly, or JavaScript effecting the layout after it has loaded.
You can use Google Lighthouse [1] to measure this (and other things) on a site.