I'm backing up from my MBP to a Time Capsule over wireless. This was lightning fast until Mountain Lion. Have done Spotlight re-indexing, updated to 10.8.1, formatted the Time Capsule, ran TMdiagnose, tinkered with Airport settings, all that jazz-- no luck.
The 10.8.2 update was no help for me, either. I just updated and started a new TM backup 3.5 hours ago, and I'm still at "Preparing Backup." The whole process, from finding the target disk to preparing backup on thru completion is still just crawling.......
I talked to Apple support weeks ago, was escalated thru their TM specialists to engineering, to whom I've sent several megabytes of log data.... no progress yet, but it seems they are aware of the issue. Must be one heck of a bug since it hasn't been addressed in two point releases so far.....