Related to another knowledge base article, Safari will create thumbnails of visited pages and create a cache to display in the Top Sites view. This is a real resources and system drag. To prevent this, we just block Safari from writing to the cache directory it places these images in.
First Empty Safari Caches by going to Safari>Empty Cache… menu item. Then go to ~/Library/Caches/com.apple.Safari/Webpage Previews and select this folder and Get Info. or Command-I and lock the folder as shown below.