ATIH2011PP: Full backup at shutdown lasts very long

I am using ATIH2011 PP build 6868 to make daily backups at shutdown of my Windows 7 system partition (holding ~200GB) to an internal disk holding the Acronis Secure Zone with a size of 2TB. Backup scheme is set for a full backup after 6 incremental backups. I set the task to keep no more than 5 backup chains. Compression is normal, priority high. No validation.

Incremental backups last about 10-15 minutes depending on the changes on this partition. That's OK. The full backup at the beginning of a new backup-chain takes 6 (!) hours to complete! When I backup the same partion to an external disk (via esata) this takes about 40 minutes for a full backup.

Why does the internal full backup last so long? There is plenty of space in the Secure Zone (~1TB), dma is on. It should only take a few seconds to delete the oldest backup chain and ~40 minutes to start a new chain with the full backup. What is going wrong here?