5GHz Speed Drops By Half Over 2.5GHz

Question

I recently moved my Time Capsule to 5GHz due to the wireless access point density in my area. I did notice greatly improved signal and reliability.
However, I noticed my speed dropped from 20Mbps to about 9Mbps.
My Time Capsule plugs into my Cable Modem, which is also a 2.5GHz wireless access point (part of the problem), when I switch my Mac to the 2.5 GHz Cable Modem, my speed returns to normal. When I switch it back to the Time Capsule, it drops by over half. The TC is being extended by an Airport Express (capable of 5GHz as well).
Obviously this is unacceptable. Is this how the 5GHz range works? It compromises your speed in exchange for better connectivity?
The Time Capsule is set to select its 5GHz channel automatically. Could this be part of the problem?
My iPad and iPhone both access the 5GHz TC, are these pulling the network speed down? They dont pull the speed down in the 2.5GHz mode.
Can anything be done to optimize the connection to restore my speed, or do I have to drop back to 2.5GHz connection.

Answer 1

It might be helpful to understand that there are always trade offs with any design.
While 5 GHz signals are capable of higher speeds than 2.4 GHz, the higher frequency signals are not as strong as 2.4 GHz signals. So, any obstructions in the signal path will affect the 5 GHz signal more than a 2.4 GHz signal.
For that reason, it is a good idea to think of 5 GHz as a line-of-sight deal. It will usually work well in the same room as the wireless router, but once you turn the corner and head down the hall, the 5 GHz signal strength will fall off rapidly.
This may likely be what you are experiencing. Your post does not provide information about where the TC is located or where other devices are located and what obstructions may be present in the signal path.  So, we're guessing a bit here.
My iPad and iPhone both access the 5GHz TC, are these pulling the network speed down?
The iPad is capable of connecting to a 5 GHz signal, but the iPhone is not....it can only connect at 2.4 GHz.
Can anything be done to optimize the connection to restore my speed, or do I have to drop back to 2.5GHz connection.
All you can really do is try to minimize obstructions in the signal path. Give the TC as clear of a look into the room as possible and each wireless device as clear of a look at the TC as possible.
The same rules would apply to an AirPort Express that is set up to "extend" the wireless signal. You might need to move the Express closer to the Time Capsule so that it will pick up a stronger and faster signal to "extend".
If you have the ability to connect the AirPort Express using an Ethernet cable back to the Time Capsule, the Express will provide a much faster signal that way since if receives full bandwidth over the wire. If it extends using wireless only, it can only extend the quality of signal that it receives. In other words, if it gets a slow signal, that is what will be extended.