The MacBook Air continues to have horrible wifi performance. I decided to replace the Apple AirPort Extreme (from 2011!) with an Asus RT-AX3000 V2 in the hope that new hardware, in place of new information, would solve the problem. As you might guess, the Asus provides network speeds 10x what the AirPort did. The MacBook Air wifi is already much better. However, the Air's wifi performance would degrade over time so let's see how it performs over the remainder of the week.
Update: MacBook Air pairs nicely with the Asus.