Hi there,
the simplest way to check what is going on is to open the site in Chrome, Right Click > Inspect – go to the Network tab in the developer tools – beside the Disable Cache checkbox, you’ll see Online – change this to Slow 3g. With the developer tools still open refresh the site.
You’ll see on your site that your Header/Nav is being initially loaded without any styles, whereas as scrolling down other parts of the site has. This is because of the Optimized CSS option in Litespeed…. unless you know how to manually write Critical CSS i would suggest you disable that feature.