Australian Review: OS X Yosemite – It’s time to buy a Mac
There's honestly not too much to report in terms of performance changes. The two Macs I've tested Yosemite on, a 2013 MacBook Air and a 2013 Mac Pro, are just as snappy as they were previously. My MacBook Air hasn't had any noticeable decrease in battery life either.
It's possible that the added transparency will be a little taxing on older Macs, but it's possible to disable this if it's causing any issues.