Ian, thanks for your post.
I just tried this this morning. I had no other applications open at all. I shift-started Mail.app, and then initiated a Rebuild (I only have 1 mail account setup in Mail.app), and nothing happened, everything went blank (so I checked the connection (good) and then I just clicked on "Sent", and then back to "Inbox" and all my mail showed up...so, just an odd bug there and a heads-up if anyone tries this and that happens.)
However,
I quickly realized that my memory dropped to 27 MB. So I closed Mail.app. Did a Memory Clean, and now I'm back up to 2.95 GB with ONLY Chrome running. I have 8 GB of memory on this system and since upgrading to Yosemite it has not had more than 4.5 GB free (typically hovering between 1 and 2 GB free). I know 8 GB of memory isn't a "monster system" but honestly, I've never had problems with this iMac EVERRRR. I run 3 or 4 Adobe applications simultaneously, with Mail.app, and Chrome, Spotify, and all the background apps and I've been doing this for 4 years and I've never even experienced slow down...now this.
So, this weekend, I'm going to try and Clean Install. A friend of mine is just raving about how great and fast Yosemite is and how he has no problems whatsoever on his Macbook Pro (but, he did a clean install.)
A couple of good Clean Install related articles:
- http://osxdaily.com/2014/10/18/clean-install-os-x-yosemite/
- http://osxdaily.com/2014/10/16/make-os-x-yosemite-boot-install-drive/