The page builder (Live Composer) is very slow when I am making changes to my website. Why is this happening?

Most likely you are using the Chrome browser. This slow-down happens because of the browser extensions/add-ons. These extensions inject their own JavaScript code on the page slowing down the  Visual Composer page builder.

There is an easy solution for that: Incognito/Private Window:

Please, try to work on your site in an incognito window and you will see the difference.