Thanks for the info. Just to make things confusing, Apple produced two versions of the A1143.
The first was limited to a top speed of 100 Mbps because the Ethernet port for the Internet connection was capped at 100 Mbps. This model was first introduced in January 2007 and discontinued in August 2007. Although you might think of the Airport as "Vintage", Apple officially classifies this model as "Obsolete".
The second version of the AirPort Extreme A1143 was introduced in August 2007 and discontinued in March 2009. On a great day, under the right conditions, the WiFi speeds might reach the 200-250 Mbps range. Apple has also officially classified this product as "Obsolete".
What does the "Obsolete" classification really mean? It means that Apple will not help you or provide support for the product even if you are willing to pay them. But, since we are not Apple, we'll try to help.
Before we do that though, you should know that the average useful life of a WiFi router designed for home use is about 5 years. Even if you purchased the AirPort Extreme as the end of its production run, the product is at least 9 years old. As such, I would expect that the AirPort will likely only be capable of providing maybe a third to half of the performance that it could when it was 2-4 years old.
As far as picking the right "channel", if the AirPort is functioning correctly, it will automatically scan and select the best channel to use. While it is possible to manually select a channel, this is a lot more a guessing game than science, since other WiFi routers in your area are also changing their channels all the time.
In other words, if you spend an hour or two trying to manually select channels, you might find one that works a bit better than the others, but a few hours from now, it won't......because another router near you is using the same channel. That's why you want to use the "Automatic" channel setting on your AirPort in almost all cases.
Frankly, it is amazing that your AirPort is even functioning at all. You have had very very good luck with the product. We might be able to suggest a few things, but I doubt that they will help much to improve your speeds.
For now, power off the AirPort Extreme for a minute or two and then power it back up. This will force the AirPort into a new channel scan, and it will pick the best channel to use automatically......assuming that it is working correctly.
Give that a try and see if it makes any difference at all.