Unless you're putting hundreds or thousands of objects on the map, the difference is not going to be noticeable. There is a slight overhead for having multiple objects, but it's not very large. Roll20 will also automatically break up any image large enough to stretch off the screen, which lets it optimize performance. If you're running any API scripts which utilize the findObjs or filterObjs functions, you should probably consider reducing the number of objects on the map where feasible, as they can be slow if they have to iterate over too many things. (Of course, this also depends on how many properties you pass to findObjs or how complicated a callback to pass to filterObjs.)