Recently I moved to a new apartment and found that the wifi network is very slow on my macbook air (mid-2011, with Mac OS X Mountain Lion), but all other devices including Windows PC, iPad and iPhone are normal. Then I connected the Macbook Air to the router with a USB network adaptor and ethernet cable, but still very slow, quite strange. Also, I booted to Windows via boot camp, also very slow. I googled it, and found many people have similar problem, but no solution... Because it happened to my macbook only, and I think it is some compatibility issue between the router and macbook air. The router is from motorola and I think it is a cable modem. Finally, I decided to change some settings in the router, to check if it possible to solve the problem. Finally, I found it, the firewall setting. By default, the firewall protection is enabled. After I disabled it, problem solved. I share this experience and it may help someone else. And I also wonder that does anyone know the reason? BTY, I live in Mexico City now.