I think it may be subject to when you "compress" the image.

But an image at 8mb with noise (grain) is equal to a crisp image at 8mb.

Else - you could state, An image with more White takes longer to process. Or with bright reds for that matter.

I also Question "Low ISO" because it's high ISO that causes Grain.

Low ISO is used for well lit situations - High ISO is used for low light situations. (or action situations ... like sporting events)