Update (3/4/14): Mac OS X 10.9.2 seems to fix at least the latency issue—and possibly dropped connections as well, at least for most users I've spoken with... We'll see!
Update 2 (11/25/14): If you're having trouble with iOS 8 and/or Yosemite, it could be related to AirDrop services over WiFi. Please see WiFried: iOS 8 WiFi Issue
For the past few months, I've been battling my 2013 11" MacBook Air's WiFi problems. I've taken the MacBook to the Genius Bar twice, and have attempted dozens of fixes. Judging by the number of individuals who have posted to this thread on Apple's Support Communities forum, among many other similar threads, I'm not the only MacBook Air owner suffering from WiFi issues like high latency, slow throughput, connection dropouts, and other random problems.
Here are some of the symptoms I and others have encountered: