look i tried an addon aircraft and the frames went down why
Simply said much more detail. I tried the 757 package from Captain Sim and it was just unusable. Fps dropped to about 4, and if I switching from a different view back to the VC it took ages for the textures to reload. Higher quality sounds, probably more polygons in the model, more advanced animations, higher quality textures, the list is endless. I do sympathize, it's a shame such great addon's just aren't a viable option with FSX for some.
But! I have recently bought "Fly the Airbus Fleet" from Abacus, and it never affected my fps at all. Though I must say, the texturing in the VC is just.. disgraceful. It's as if a photoshop novice came in and did it for them, it's all pixelated and 2D looking. Other than that I'd say it's great. If MS went into more detail with airliners there would be no problem, but they deliberately won't so these other companies like Abacus, Captain Sim etc can make the addon's. Therefore MS get a percentage of the profit plus money from licencing etc. Lol the whole thing is shrewd business.