The size of my MK7 romfs.bin file is just 602 megabyte. After dumping there's also another file called exefs.bin, which is much smaller (3,50 mb for me). I don't know what this is used for. So the dumping itself was done quite quickly.. however: If you have a New 3DS, you should work on your ROM outside of your sd card if you don't want to experience loading times all the time (since you can access the card with internet transfer "only"). Copy the romfs_ to the desktop. You have to wait once to copy everything, but then the workflow is much better.