RAM isn't going to help you, it depends upon the hard disk RPM speed and also how much has to be moved around. You can defrag the fragmented files only from the File list if you don't wish to also optimize the disk -- since optimizing ("filling gaps") on a 7TB disk with 7GB free space could take a very long time.
- You indeed can speed it up. Move anything between say 10% to 25% of the drive's content to another drive, defrag the drive and then move that previously moved content back. When you move the content back then the OS will try to keep the defragmentation as low as possible.