Time Machine Backups

First TM ever to the TC ?

If so ... Try doing first backup via ethernet not wifi since the largest backup in the "set" .. every backup after should be quick since TM is only backing up the "changes" said files ..

I'm having this issue as well, though my last backup was back in July right before I installed PB 1. For mine, I was thinking that it was because of it being a long time since my last backup, but since it has been only a week or so since your last backup, it must be a PB problem.

i had this same issue right before upgrading to PB3, so not sure why its happening

Is it possible that installing Yosemite, your partition was changed to be a CoreStorage Volume, which has a unique ID. This unique ID might make TimeMachine think that you are backing up a new machine, thus making a full backup?

Try reverting your drive from core storage. There are many threads on how to do this.

I'm having the same issue. It's taking ages by USB 3.0. Seems to be a Yosemite problems. Hopefully it's solved in the next DP or PB