As already explained in this forum, FS9 doesn't support multicore systems, and this is much apparent for texture loading, since it's done on the same core as the main thread so, either the simulation is slowed down when loading textures, or the textures appears with a delay. As you noticed, the FSX version doesn't have this issue, because FSX is able to load textures in the background using other cores that always have spare processor time to use without slowing the main simulation.
You can try with the textures resizer posted here.