The most common use of JavaScript is to load pages with JavaScript. There are a few reasons why this is bad. First, they can take long to load and thus slow down the page. Second, if the page is very large, then not all of the pages that are loaded will be loaded immediately. So you end up with a lot of pages that load slowly and don’t do anything except slow down the page.
JavaScript on a page is not necessarily a bad thing though. As long as the script is loaded and executed quickly enough to avoid slow page loading, you should be fine with having it.