This shows a terrain consisting in 16 x 16 tiles of 400 units long. The tiles are loaded asynchronously as the camera moves around (although there are still some serious slow-downs !).
On the right you can see the array of tiles where "?" and "X" represent the tiles registered for loading and the loaded tiles respectively.
Loaded Tiles
Use Alt+Mouse to manipulate the camera using MAYA mode.
Use Alt+Shift+Mouse to manipulate the camera using UnrealEd mode (i.e. first person).
If you are under Linux and can't use the Alt key, push that button so everything works without Alt.