As far as I know it not only influences the loading time. It also influences the transfer to the graphics card and thus can have an impact at rendering speed when there are many switches between different textures.
I'm not sure if we're still talking about the same thing.
If you're talking about multiple textures vs. single texture you're right.
I was talking about single "empty" texture. Apart from DDS the engine uncompresses any compressed format anyway internally which my result in different loading time for "empty" and "filled" textures. But rendering time should be the same.
