You may be using older versions of scripts that a customer's browser is waiting to load before rendering the remainder of a page.
Older scripts were synchronous which means they had to load while you rendered the page. New scripts are asynchronous which means they will load in the background and will not delay the rendering of a page.
Your web developer can review any scripts on your page (i.e. Facebook, Google Analytics etc.) to ensure they are all asynchronous.