How long have you had Mountain Lion installed? How many backups has TM made with ML? How much space is left on your external drive?
I ask these questions because I found that for about a week, TimeMachine was running slower than it had under Lion. It seemed that the preparing backup stage took a very long time and then the first few megabytes were slow. But the backups are going much faster now - as fast as Lion was - though the preparing stage does still seem to take longer.
Another thing I noticed on my notebook which has a smallish TimeMachine drive is that the thinning process (erasing when more space is needed) does seem to take longer which results in a slightly slower backup. This was true of Lion too - but I think a ML backup that requires thinning does take longer than one with Lion.
This is happening to me as well, it's telling me 15 hours left to finish backing up the 400GB of data on my drive and that amount of time keeps increasing. It is moving files but this is insane that it is taking to long and I doubt and this time it will even be able to complete successfully.
There are numerous threads on this all over the place, another serious bug I think in ML.
I found the way to fix this on the message boards (thanks to Swiggen).
Check if your spotlight is indexing. If it is, let it finish, even if it takes days before plugging in your ext hard drive.
Mine didn't seem to be busy indexing, so I stopped the indexing function (through Terminal) and then restarted the function. It then indexed for 2-3 days.
Once it was finished, I plugged in my external hard drive and Time Machine started up and did a backup of 118gb in a few minutes.