The next update to OS X 10.8 (version 10.8.5) is expected to be released imminently, and among a number of included stability and performance fixes, Apple has addressed a throttled networking speed bug that affected systems with 802.11ac Wi-Fi cards.
Apple released new MacBook Air models at WWDC that, along with impressive battery performance, contain faster 802.11ac-capable Wi-Fi network adapters, offering real-world transfer speeds of around 50MB/s. However, in testing, some noticed that the speeds of these systems were truncated. While transfers were still fast, they topped out at around 21MB/s, under half the expected speed.
Investigations by AnandTech revealed that the problem was in the TCP window size used with the file-sharing protocols in OS X; a maximum of 64KB was used instead of the ~256KB needed to maintain the higher throughput of the 802.11ac connection.
If you have one of these systems and are frustrated by the truncated networking performance, expect the OS X 10.8.5 update, which is rumored to be out today or sometime in the near future, to address the issue. The update implements the fix to the file-sharing protocols, bringing speeds up to their expected ranges.
In addition to networking fixes, the 10.8.5 update addresses some problems that prevent Mail from displaying messages properly as well as issues in which screensavers don't start, and it improves XSan storage reliability.
When available, the update should be accessible through Apple's Software Update service in the App Store, which can be accessed by choosing Software Update in the Apple menu. As with all point release updates for OS X, this will heavily modify your OS and require your system to be restarted, so in preparation and out of precaution, be sure to set up and maintain a full and restorable backup of your system using Time Machine or a system cloning tool like Carbon Copy Cloner.