Tip: Increase Internet speed by manipulating MTU size
The MTU handling in OS X Mountain Lion may not work with the configuration for some Web servers, and might need to be adjusted.
Knowledge Base: Setting the MTU value; Ethernet speed; Windows AFP servers
Changing MTU setting fixes problems sending email
Adjusting some Flash and system settings may help when Flash Web content hangs or shows choppy performance.
The new MacBook Air has Wi-Fi issues -- so say owners in Apple's support community. This wouldn't be the first time that a new Apple product had Wi-Fi hiccups.
Adjustments to certificate handling for some captive network setups may help overcome network access errors or blocks that prevent e-mail or Web access.
Yet more tips for boosting Safari's performance.
Apani Nortel Contivity client issues under Mac OS X 10.4.5
In addition to using the system preferences, you can adjust Wi-Fi network configuration in OS X via Terminal.