I have the first haswell entry level MBA with 4gbs of ram. Occasionally i experienced beachballs with safari on mavericks.... But with yosemite after a while of usage (after quit safari or reboot) i get beachballs all the time....the problem is that even when i close most of the tabs and i leave 2-3 the beachball issue doesnt gets away.... only solution is to quit safari and reopen it..... until the new beachball will occur..... It driving me nuts... apple should never ship a Mac with less than 8GB.... except if they have the time and the balls to optimize the system like devs did in the 70es with Amigas before Moneysoft era. Apps i always have opened are (viber,itunes,mail,tweetbot and radium... i only use safari on full screen) Apple should at least give us the option to upgrade the ram for gods shake.