"I'm running Logic X on the clean install I did for OS X.10 Yosemite. (About 30 hours of work.) Logic Pro X is working with perfect performance on my 2008 Mac Pro. (IIRC, Dual 2.8GHz Quad-Core Xeon.) So, don't know why someone else would get bad CPU performance on their Mac using Duet 2. (Duet 2 not the issue per later comments/tests with it removed. He has a 3.1GHz quad-core i5 iMac.)
I have an Apogee Ensemble and a Duet 1. No CPU problems here. I'm sending screen shots of a song I'm working on with 33 stereo channel strips and 22 AUX tracks, using tons of plug-ins and I'm showing very low CPU usage on this relatively huge song.
1) WaveBurner and Soundtrack Pro do not work in Yosemite, when they used to work on Maverick. It appears to me that Apple has really created a difficult permission code for audio on Yosemite. (He sent a large screenshot of WaveBurner crash report, headline "WaveBurner cannot be opened because of a problem".)
2) Snapz Pro Report: (follow-up to prev. comments on v2.5.4 audio capture problems)
Reply to Ambrosia Software Support request: "We are aware of the incompatibilities between Snapz Pro and Yosemite and while there is an update planned for the near future we cannot give you an ETA of the release."