Airport Extreme Running Slowly

What cable modem do you have? Is it a router?

The airport AC model has major issues with its WAN port.. it sounds faulty to me. But it is also problematic with certain cable modems.. SB6121 and 6141 in particular give trouble.

If Charter has given you a modem router.. called gateway in some cases.. use that as the main router and the AE should be bridged.. in bridge mode you can use a LAN port on the AE instead of WAN.. give it a try..

Otherwise take the AE back to apple store for testing replacement.

It is a cable modem

Unfortunately, that does not really provide us with much information. As LaPastenague mentions, there are some problematic modems that simply do not work that well with the AirPort Extreme.....or you could say that the AirPort Extreme does not work well with some modems. If you could provide the make and model number of the modem, that would really help us with troubleshooting things.

when I plug my iMac straight into the modem, it gets speeds between 80 and 100 Mbps

The modem seems to be working OK. But, you did not indicate what speeds that you measure when you connect the AirPort Extreme to the modem and then connect the iMac directly to one of the three LAN <--> ports on the AirPort Extreme using an Ethernet cable connection.

Remember that you first need to make sure that Ethernet connections are working correctly at normal speeds.....before.....you can begin to troubleshoot a wireless connection. For example, If the WAN port on the AirPort is having difficulty....a common issue with the AirPort Extreme.....that issue will need attention before your begin to troubleshoot the wireless. Once you get the Ethernet speeds up to normal, the wireless speeds may improve as well with no additional attention.

IF.....you get 80-100 Mbps when the iMac is connected directly to the modem, then you should get about the same speeds when the iMac is connected to one of the three LAN <--> ports on the AirPort Extreme using a wired Ethernet connection.

Is that about what you measure when you run a test that way?

I have my MacBook Air and iPhone one floor below the router in the room directly under it.

Then, you cannot really compare those speeds to a direct Ethernet connection.

You know that an Ethernet connection comes in at say, maybe 90 Mbps on average.

Now, in order to make a meaningful comparison with wireless, you need to locate the MacBook Air about 8-10 feet from the AirPort router with line-of-sight between the two devices. Since there will always be some loss with wireless, you might expect to see speeds in the high 70s to low 80s when you run this test if the router and Mac are working reasonably well.

What speed do you see if you try this test?

Remember that the wireless speed will fall off significantly the further a device moves away from the router or if there any obstructions in the signal path between the router and computer. A ceiling......which is much thicker than a wall.....might slow things down by 40-50% on average. This is normal.

I would pay no attention to the speeds on an iPhone or iPad. These devices use a much simpler operating system and a much simpler antenna compared to a computer. As long as the iPhones or iPads are operating OK, I would not pay any further attention to those devices. For example, I can measure a 100 Mbps wireless connection with a MacBook Pro. My iPhone might come in at 40-45 Mbps at the same location on a good day, 30-40 on another day. The iPad might do a bit better....maybe 50 Mbps.

Finally, remember that wireless always has been, and always will be half science and half voodoo. Some days will be better than others.