AirPort Extreme seems to be malfunctioning, but so does my AirPort Express?

If you get 30 Mbps with a wired connection, then it would be normal to expect to see about 25 Mpbs or so when the computer is located 8-10 feet away from the wireless router with direct line-of-sight to the router.

If you power off the AirPort Express temporarily, and then power off and restart the AirPort Extreme and run a check, what speed do you see when you run this test?

If you are getting the same results when you test with another computer, you are picking up some strange wireless interference from other networks around you....or....the AirPort(s) are defective.

See this Apple Support document for info on wireless interference issues: Wi-Fi and Bluetooth: Potential sources of wireless interference - Apple Support

Take the AirPort Extreme to an Apple Store....if you have one near you to let them take a look. If the AirPort performs well there (it likely will), then you know that you have some real interference issues to deal with.

I tried to help a friend recently who moved into a new development, and there was so much interference there due to other networks, that he cannot even use wireless at all. He would be happy to get 3 Mbps if it worked.

What have you tried so far?

Shut down the Mac, wait a few seconds, then power it back up and try again. If no luck, go to the next step.

Open System Preferences (gear icon on the dock)

Open Network

Click on WiFi on the left side of the window to highlight it

Click Advanced at the lower right of the window

Here you will see a list of networks that the Mac has connected to in the past

Click on the name a network to highlight it, then click the - (minus) button at the bottom of the list

Do this for each listing that you see until the box is blank

Click OK

Click Apply in the next window

Shut down the Mac, wait a few seconds, then power it back up and try again, If no luck, go to the next step.

Take the Mac to an Apple Store or Apple Servicer to have it checked out.