I think a lot has to do with texture loading. If you fly over a smaller city you can load everything at once, but flying over London with a lot of buildings your video RAM is not enough and the textures are loaded in batches.
For example, if i use maximum detail on my older Ati 4890 1Gb, i can clearly see that even inside the cockpit sometimes the textures remain low-resolution and don't load immediately. That's because i have an older GPU with less video RAM and i also have only 3Gb of RAM, so the sim loads whatever it can first and then loads the rest in "steps".
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