[Edit:] And I see up to 13 fps if I fly away from the sun, the author seems to be right when he writes "Calculating sun visibility the geometric way (by casting rays through the terrain and the clouds) can be expensive if not done carefully."
$ uname -a Linux XX 3.5.0-17-generic #28-Ubuntu SMP Tue Oct 9 19:31:23 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
edit: Ok, I figured it out, it's because I used an external 1920*y display, if I resize the window it gets faster.
I also tried it on my 'mac' desktop with is an i7 920 @ 4.1Ghz with a nvidia 660Ti running OSX, also using chrome and with all windows open it only does ~40-45fps but pegs at 60fps again with them closed.
Resolution probably makes a big difference also, on both machines I have the chrome window 'maximized' but the desktop has 1920 x 1200 monitors vs the air's 1440 x 900 screen. I'm not sure how webgl handles retina resolutions, it's also possible the nvidia gpu isn't kicking in for some reason, assuming your on the 15" rmbp.