Interface (UI) feels very fast and responsive in the PTE Build

Thanks!
It just feels much better.

I wish I could share that observation, its still lags like hell on my system, regardless of what graphics settings (and I am on the 71400 PTE build). Is it an OS X related issue? Other than that, game is looking amazing

Hmmm, sorry for you. I play on Windows 7 64bit.

I don't want to rain on your parade, and before I will, I have to mention that I feel those improvements as well, fps and UI responsiveness sure feel much better.
But. For me, it was exactly the same during last PTE, one that introduced new nuke and commander explosions. When I first launched that PTE, I was amazed at performance improvements. However, when build went stable, performance was better, but not that better.

Maybe it's because of different server, maybe because I use steam for stable or anything else. After that time, I prefer not to keep my expectation that high. Which is a win/win: either I will get little boost I expected, or huge improvement that will make me jizz all over my monitor (as opposed to huge boost 'as expected' or huge disappointment if you keep your expectations high).