incremental rdiff-backup takes long

Hi all!

I found out about rdiff-backup two weeks ago after I searched for a
method to get incremental backups of my system.
I experience some problems while doing my backup and I hope that
somebody can tell me what I am doing wrong.

I understand that the initial backup is really slow and took more than
~10h for my 400 GB.
Now if I do another backup, I would expect that it is quite fast, but
after it exceeded 8h and I just aborted the whole backup process.

I use the following command:
> rdiff-backup --terminal-verbosity 8 --print-statistics \
>    --exclude-globbing-filelist $HOME/.excludes \
>    $HOME $DRIVE

Can anybody tell me what I am doing wrong?

Thank you for your help!

Cheers,
Jonas