Time Machine is faster on Mountain Lion and Lion than on Snow Leopard.
What is your backup configuration?
You may need to just start over.
Have you perused Pondini's Time Machine site: http://pondini.org/OSX/Home.html
Thanks for trying to help, but belive me, I wouldn't be that mad if I haven't tryied everything before. I wasted days by re-setup machines, re-configuring and try it over and over again. The only thing that is working as it should do, ist the local Backup on my server. Everything over the network (ethernet and/or WiFi) failed, or getting close to it by taking weeks.
Finaly I now found a solution - I turned my Ethernet connection from automatic to manually - that's it...!
This is clearly a ML-Bug. Erverything was working fine under Lion. I hope now, this configuration dosn't harm my usual network flow.. we will see.
Sorry for my bad English.. it getting worse when I'm angry
OK... here is an Update - after I switched from Automatic to Manually everything was working fine. The next time TM trying to make a Backup, it went down to abput 80Kb/s again. The Problem is within the Ethernet Connection - if you disable it for a second and re-activating it, guess what - it runs at full speed agin... up to the next time... please APPLE... FIX THAT BUG...!