Hi,
Could you try to play with p3d radius setting? maybe by lowering a little bit the value. I do not think is a problem of the scenery ( because the scenery has nothing to load once you are using it ) but maybe could be some object ( non part of the scenery ) which take some time to load.
Anyway i will try to find if i can understand better the cause and a method to solve it, it is not easy because i do not have the pause at my end.
Cheers