Time machine too slow

After waiting about three days for TM "prepare" the initial backup on Time Capsule, I made the initial backup vie Ethernet hoping that the "incremental" backups would be timely. Now, it took around two day to transfer 25 MB of 29 MB. Currently, TM is preparing another backup for some hours.

Since installing Mavericks, no other drive on my network is being offered for TM. It only asks for Airport. Thus, I bought one. The outcome is written above.

Where is the culprit? (I have two other machines with Mavericks with no problems with TM)