This has been discussed many times: you can act on the Anti-pop up slider in the Addon Manager to control loading distance.
Low values will result in fast fps, at a cost of some stuttering when objects are loaded/unloaded, while high values will load more objects at the same time, at a cost of more memory taken, and likely lower average fps, but less or no stuttering. At the maximum value, everything will be loaded at the same time when the airport goes in range, so there will be no further load/unload.
You don't see this in other sceneries, because they load everything at the same time, this is the only way allowed by usual FSX SDK methods, so it would be similar than having the Anti-popup at the maximum value. We offer a chance to control the process, because the correct value depends on the system used and the load caused by other installed products.
