The Addon Manager can't cause any stuttering due to texture loading. It doesn't know about texture handling at all. This is done entirely by FSX engine.
Starting with the fact that not everyone see it, stuttering might be caused by a lot of things.
First, other "high quality" scenery might not use FSX texturing at all, but it can simply be FS9 visuals used in FSX. It's quite obvious that, a simple diffuse texture not only takes much less space in memory than a texture with diffuse+specular+bump, but also its shader can be slower to execute or, better yet, triggering a state change that *might* cause stutters, if the graphic settings are put too high (the system, being already overloaded, can't process drawing calls fast enough). A scenery with a flat FS9-looks, that is drawing everything with the same shader, will not have this problem. But then, what's the point of using FSX...
Other problems might cause stuttering, is the added load of AI airplanes, especially legacy FS9 models. This, when used in *combination* with a scenery that instead uses fully compliant FSX 3D objects (like we do) forces FSX to keep switching between its two different rendering paths, the FS9 and the FSX one. If one does everything FS9 style, the problem might be lessened. Of course, only until a fully FSX compliant AI package comes out...
Another reason for stuttering might be exhaustion of the Video RAM. This can happen on cards with less than 512MB, especially when used in combination with AI packages with lots of *different* liveries. The constant switching between AI models textures COMBINED with the airport scenery textures might cause the video card to struggle, the less VRAM the more the struggle. Solution here is the usual: LOWER your settings, the most important ones are the Global Max Texture size (decreasing it one notch will lower memory requirements 4 TIMES! ) and the Scenery Size, if it's put to "Large" you really need a lot of VRAM to handle the scenery.
Other reasons for stuttering might also be external ones, not related to FSX. For example, a big issue is Vista file indexing and automatic defrag. That is the N.1 source of problems with anything related to disk loading. If you constantly see your hdd light flashing while you are flying, most of the times it doesn't have anything to do with FSX, but instead is the Vista file indexer and the automatic defrag that is running on its own. I always shut down these services, it really makes a lot of difference in smoothness.
Of course, all this doesn't have anything to do with the Addon Manager. The fact you are seeing with our sceneries more than with other, it's only because we use very high detail textures, with many of the advanced FSX properties.
