Images will load slowly the first time regardless of Design Tools use. This is why browsers cache images for faster retrieval later. In general, Canvas will load its interface and then pull in user content. This means Design Tools has to watch for the content to load before it knows if it needs to create components, like tabs or module details, resulting in a slight flicker. Adding an iframe into a tab or an accordion and it will delay even longer to make sure everything loads correctly before being hidden. With Module Details, Design Tools has to ask Canvas about the modules before it can build the interface so there will be a slight delay while it waits for a response.