Best Wifi channel and 5GHz

The Time Capsule has two Wifi frequencies, which can be the same network or can be given different names. There is a tendency for equipment to default to the 2.5 GHz option, so sometimes it's necessary to use the separate name option to avoid this.

2.5 GHz is more powerful and robust, but is more popular and so you tend to see a lot of other networks. Sometimes these can cause interference, though mostly the fact that a lot may be showing doesn't really matter.

5 GHz is as you've found noticeably faster, though not as strong or robust, though you will get less trouble from interference since not only are fewer people using it, it doesn't travel as far. If you are getting a good stable signal with it, it's the preferable choice. Only if you are finding it too weak should you revert to 2.5GHz.

I don't think it's a good idea to post your Mac address and I've asked the Hosts to remove the image.

why is it off by default?

It's not. By default, both the 2.4 GHz and 5 GHz bands use the same wireless network name. Devices should choose the best signal with which to connect depending on their location and capabilities.

For example, if my Mac is in the same room as the Apple router, it connects at 5 GHz. If I move the laptop a few rooms away, the laptop connects at 2.4 GHz.....since the 2.4 GHz signal is much stronger at that location.

If a device is only capable of connecting at 2.4 GHz, then it will of course connect at 2.4 GHz no matter where it is located.

If you take a look at the 5 GHz "option" in AirPort Utility again, you will see that the option doe not "turn on" 5 GHz. The option allows a user to assign a different name to the 5 Ghz band.

If you use the option to assign a different name, you can in effect "force" a device to connect at 5 GHz. This works great if the device is in close proximity the router. But.....it does not work nearly as well when the laptop moves a few rooms away from the router. The device will try to stay connected to the 5 GHz signal.....even though it is much weaker (and also possibly slower) at that location.

So, the bottom line for most folks is that wireless is always a compromise. Nature of the beast.

There is less interference on the 5 GHz band from other networks (simply because there are fewer of them and the 5 GHz signals probably won't reach you) so if you can use 5 GHz.....and keep your devices close to the router....that is probably your best bet.