"When Patch 1.5 came out in WoL, Blizzard changed the way it patched games. It has 3 stages (Unplayable, Playable, and Optimal). So now when a map loads, it pre-loads all data that it may need (units, textures, effects) whether they are start on the map or not. The "streaming" part is actually just accessing stuff off your hard drive BUT if something was missing (ie you were playing the game at only the "Playable" level of download, not optimal) it would attempt to download it from Battle.net. Pre-1.5 data would not be loaded in the same way, it was actually much faster, but it also wasn't able to stream data from the internet if needed."