I replicated the experience reported by Barabas2005
My machines are connected to the network at 1000baseT speeds. However, when Timemachine starts on the Mac Pro the speeds are only around 80KB/s. While the back-up is still in progress, I switched to the WiFi network. All of a sudden the speed jumps to 5MB/s! When going back to the wired network connection, it even jumps to 30MB/s and the back-up finishes very quickly (like it always did before upgrading to Mountain Lion).
However, when Timemachine starts its next session after an hour, it goes back to the very low 80KB/s speeds.
and I got the exact same results.
Someone gave me a 2007 Mac mini; I set it up with Lion, I connected it to my network and I connected my external drive to it. Now I can do the Time Machine backups of my two iMacs on that host at full speed. In this manner, I solved MY problem, but the original problem is always there.
This and all the other experiences reported here by other users clearly demonstrate that there is a real problem with Time Machine backups when the backup drive is on an external host running Mountain Lion. This problem is there since the debut of Mountain Lion and it's a shame that it has not been corrected after two updates.
But is Apple aware of the problem? I'm not sure and I don't know how to report such a problem directly to Apple. Someone knows?