I would guess that the beach ball you are seeing is the generation of the 1:1 preview or how long it takes to calculate the entire raw instead of a lower res preview. The display on these new iMacs is so gigantic that the standard preview sizes are way too small (the max is 2880 pixels long, which is a factor of 2 too small for the retina display). The retina machine has 4x as many pixels as the older 27" iMac. The difference in speed between 4.0 Ghz and 3.5 GHz is not enough to make up for that difference in number of pixels that need to be calculated from the raw to make up for that, so you should see some decrease in speed if you use Lightroom full screen. Try making the Lightroom window about 1/4 of the screen and see if that changes things.
Ah, I wondered if it might have something to do with the retina display. I tried what you suggested and made the LR window about 1/4 size of the display. I was then able to apply presets instantaneously, with no lag at all (much faster than my previous system). I also opened some images in Photoshop and the Nik filters, and reduced the window size. The Nik filters are still a bit more "laggy" than I would expect, but overall there was an improvement.
Is Adobe planning on updating the max preview size given these new displays?
I wish I would have considered this prior to purchase. Probably wouldn't have stopped me, because the retina display is so stunning, but the lag is really annoying.