I was about to create a thread complaining about UI performance on the new Macbook (1.3 BTO). When switching screens, launching safari and general usage there was without doubt noticeable lag... Especially in comparison to my 2014 Macbook Air. Now.... A small recommendation to those who may also have the same experience. I have just enrolled this macbook into the BETA for 10.10.4 and installed the update. Whilst I don't know the ins and outs of this upgrade I can confirm the UI performance appears to have significantly improved. There's no longer any lag switching between screens and launching applications. Whilst its not butter smooth, its vastly improved and almost on par with the i7 Air. I don't know if this update included any performance fixes, none the less it has improved my experience. Id recommend you do the same.