Time Machine extremely slow on Mountain Lion remote host

After the switch to Mountain Lion, the backup strategy I used for years no longer works. I have two iMacs at home, each with an external hard drive used for Time Machine backups. Each iMac backup locally to the external hard drive and remotely to the external drive of the other iMac. My network is wired through my router; all the devices use Gigabit Ethernet. Normally one of the external drives is kept out of the house.

Ten days ago I replaced one of the iMacs with a new one and I immediately installed Mountain Lion on it. After the complete installation of all my stuff on that new Mac, I updated my local backup and I erased the backup on the remote Mac (which was still on Lion) and created a new one; this took normal time, about 3 hours for 215 GB (1.2 GB/min). But when I went to update the backup of the other iMac remotely on the new one, it was very slow. I then erased that backup and began a new one. It was extremely slow (about 11 MB/min) and Time Machine said that it will take 5 days to complete the 65 GB backup (compare to the 3 hours for 215 GB above).

I thought that this could be caused by a bad interaction between Lion and its successor. So I upgraded the other iMac to Mountain Lion. After that, same problem: the local backup went normally but the remote backup was still very slow. And now the remote backup from my first iMac is also extremely slow on the Mountain Lion host. I erased that backup and tried to create a new one; this time it didn’t complete in 3 hours but Time Machine estimated that the 215 GB will be backed up in 15 days!

After that, I also tested the direct copy from a Mac to the other and concluded that all is normal on that side. The copy of a 2 GB folder containing 82 files took about 35 seconds.

So, my experience shows that

1. backing up from either Lion or Mountain Lion to a remote host that is NOT under Mountain Lion goes normally;

2. backing up from either Lion or Mountain Lion to a remote host that is under Mountain Lion is abnormally slow.

Do you share similar experience? Have you found some solution?

Hope Apple will address that issue real soon now!