The tram loading screen could have been an engine limitation. Streaming the game world from disk works nicely if the player moves at a pace that is slow enough

The tram loading screen could have been an engine limitation. Streaming the game world from disk works nicely if the player moves at a pace that is slow enough that the disk can keep up. Long range teleportation breaks that assumption completely. You can deal with that case in two ways: accept a loading screen or build teleportation support into the streaming system, which means that both ends of the teleporter need to be loaded at once if you walk up to it. I assume that Blizzard decided against all that complexity deep in the engine for a single use case.