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It's runs smooth, but battery life seems to have decreased. Not sure what it is, but I used to get 10-11 hours and now I get 7-8. Very noticeable and very frustrating.
 
It runs almost perfectly on my mid-2011 Air, with smoother transitions and faster boot/wake-up times than on Mavericks.
 
It did nt run as smooth as Mavs on my Mid 2012 4gb MacBook Air. So I reverted back to Mavs.
 
I can confirm that for me the laggyness seems to be triggered by preview.app especially with complex pdf files.
I work a lot with architectural drawings and my macs become almost unusable when trying to open such drawings.
I can reproduce the issue on my:
MBA mid2013
MBP Retina late 2013
iMAC 27 2013

On Mavericks the zooming and scrolling of the files works just smooth as butter.
Here two exemplary pdf files, I would like to receive some feedback specially from the people having no issues:

https://drive.google.com/folderview?id=0BwQwR5XgWLk2MXJUREZxUHFvVm8&usp=sharing

You can try reverting to the Mavericks PDFKit:

https://discussions.apple.com/message/27639834#27639834
 
My Mid-2012 13" Air runs just as well on Yosemite as the day I bought it. Actually, I think the battery life improved slightly with the upgrade.
 
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