How do I prevent the page from rendering the header seconds before the content?

Dealing with a rough issue here. Currently have no site optimization configured (that I am aware of) but for all pages on the site, the header loads a significant amount of time before the rest of the page content. This is causing a large issue with CLS on Google, as the page shifts when the body of the page is rendered. Is there any way that I can enforce them to load at the same time? This only became an issue recently.