If you find your wi-fi network is getting slow, there may be interference from another network. If another Wi-Fi network nearby uses the same channel, or even a close channel, the two networks will collide, reducing speed for everyone. Here are some tips about how to troubleshoot Wi-fi…
In OSX Lion or OSX Mountain Lion, hold down Option and click the Wireless network menu item at the top right. This shows the wireless menu but with some added information. Under your network look for Channel. It will usually be 1, 6, or 11. Then hover over the other networks and note the channels that they use.
If any are close to your channel you might see some improvement in Wi-Fi speed if you pick another channel manually. You need to use a channel as far from all the others as possible, and ideally it should be farthest from any channels that have a good signal(since they will disrupt you the most).
Open Applications > Utilities > AirPort Utility. Edit the settings for your device (the exact steps here depend on the version of AirPort Utility), go to Wireless > Wireless Options and select the channel you decided on earlier. Save the settings and wait for the device to come back. Then test!
This issue seems to be particularly bad when the base station (a Time Capsule in my case) is in a far corner of the house, where, for example, channel 11 is a good choice. My office, in the other corner of the house, has many other nearby networks, some of which are on or around 11. So channel 11 is actually a poor choice for a channel. I looked at the channels in use nearby: 1, 6, 9, 11, 13. I manually set my Time Capsule ( but this is the same for any AirPort base station) to channel 4, which is around halfway between 1 and 6 and seemed like the best bet.
Up to 270Mbps mode uses two channels, but in this mode only the first channel is listed in the channel pulldown menu. The associated channels in this mode are: 1 5, 2 6, 3 7, 4 8, 5 9, 6 10, and 7 11. When you select another wireless network mode, the channel pulldown displays all available channels: 1 through 13. However, available wireless channels depend on the selected wireless region.