A "direct" backup from FD into a SD tape device is performed with a little buffering and require more computation, disk seeks and context switching. It is a single, complicated chain with two threads one for FD and one for SD connected through logical network layer (even when a full backup is performed on local machine). With this chain it is very hard to get a full LTO speed. As opposite when you perform SD data spooling, you can easy achieve a full LTO speed because a writing chain (despooling) in SD is very simple and effective. Unfortunately doing a data spool takes a long time and slow down an overall backup speed.