Creating a snapshot is not the backup itself. It is basically freezing the current state of your files from this point on and then remembering what has chanced compared to the snapshot. So yes, it is normal that this takes no time since nothing is copied on your hard drive. If you want to backup the snapshot you need to make an actual backup of it. This is possible with btrfs. But since I haven’t done it yet, I can’t tell you the exact command.