“Static” sidebar?

I created a sidebar for my vertical navigation (with CSS).

The sidebar I created with css for the navigation, always has troubles loading. Everytime I open or reload a page, I first see the header and the page content (over the whole sreen), before the content jumps to the right, and the left sidebar with the menu shows up (that takes up to 2-3 seconds). This is the CSS I used for the sidebar.

.is-left-sidebar {
    width: 20%;
}

.site-content .content-area {
    width: 80%;
}

Im a noob, so I dont know that exactly. But I suppose, the header doesnt reload everytime. Is there a way, that the sidebar is handled the same way? So it doesnt reload everytime either? To make the sidebar to a “static” element of the whole website, like the header?