Large amounts of data in a layer or the map may affect the performance of the map. For example, you may lose the map you are working on if you navigate away from the map viewer when the web map is too big for your browser to cache (generally, bigger than 2.5 MB, except with IE7 [64 KB] and IE8 [530 KB]). To help prevent this, sign in before you start working and periodically save your map, especially if you add a large amount of data.