Why is my Time Machine backup so slow?

In short: A backup of 2 Mbytes takes more than half an hour

Long version

I used Time Machine with my old iMac and of course I am using it with my new iMac (SSD + 2 TB). The Backup destination is an external FireWire drive with 2 TB, the same I used with the old iMac.

The first backup took some time (more than 700 GB), but this is normal, I assume. Now I have problems with the "small" delta backups. Just one example from the console output:

27.06.11 17:56:20 com.apple.backupd[8775] Starting standard backup

27.06.11 17:56:20 com.apple.backupd[8775] Backing up to: /Volumes/Archiv 1/Backups.backupdb

27.06.11 17:57:56 com.apple.backupd[8775] No pre-backup thinning needed: 855.7 MB requested (including padding), 883.02 GB available

27.06.11 18:06:31 com.apple.backupd[8775] Copied 226532 files (1.8 MB) from volume Speedy HD.

27.06.11 18:06:50 com.apple.backupd[8775] Copied 226781 files (1.8 MB) from volume Speedy Medien.

27.06.11 18:09:48 com.apple.backupd[8775] No pre-backup thinning needed: 871.3 MB requested (including padding), 882.92 GB available

27.06.11 18:17:59 com.apple.backupd[8775] Copied 178604 files (236 KB) from volume Speedy HD.

27.06.11 18:18:19 com.apple.backupd[8775] Copied 178853 files (236 KB) from volume Speedy Medien.

27.06.11 18:24:55 com.apple.backupd[8775] Starting post-backup thinning

27.06.11 18:33:17 com.apple.backupd[8775] Deleted backup /Volumes/Archiv 1/Backups.backupdb/Speedy/2011-06-26-171728: 883.00 GB now available

27.06.11 18:33:17 com.apple.backupd[8775] Post-back up thinning complete: 1 expired backups removed

27.06.11 18:33:18 com.apple.backupd[8775] Backup completed successfully.

As you can see from the log there is no single error message or warning but the backup of about 2 MBytes took 37 Minutes !! And this happens every hour. Most time of the day my iMac is backing up!! If I add some content to my iMac, (e.g. the backup of a new TV show with 3 GBytes) the backup is as fast as expected (at least the copy phase of Time Machine). Nevertheless every backup has a long preparation and a long cleanup phase. During these ridiculous long phases the activity of the external FireWire drive is very high, I hear the head of the drive positioning all the time.

I checked all discs with disk utility, repaired all rights, rebooted the iMac and switched the external drive off and on.

The backup source consists of the SSD and the HDD of my iMac. For the HDD I disable access rights management to share an iPhoto library easily between multiple users in our family.