If you experience a slow Visual Composer this it’s always related to browser performances or extensions you have installed. We have noted a very few users with this problem and in all the case studies simple use another browser fixed the problem.
For example if you use Chrome as your primary browser, and you have a slow Visual Composer, just use FireFox or Safari. This happen cause the primary browser may has lot of cache elements in the memory or possibly you have installed extensions that may interfere with the Visual Composer functionalities.
if you use chrome then this could be the problem described in this thread