Let me start by saying that I’m horrible at understanding anything network-related in my computer setup. The following is just what worked for me in solving this problem, and may be of use to others.
My 2013 13” Macbook Air started having network speed and connection issues, primarily with WiFi, but also when hardwired through my 27” Thunderbolt Display. My older Air, now owned by my wife, was able to connect to the same networks without issue and achieve reasonable speeds.
I first noticed the issue on other people’s networks, but chalked it up to cruddy routers and low bandwidth on their end. Then I noticed it on my home network, where I have a 100Mbps line and a 5GHz wireless router (new Airport Extreme). I was seeing about 1.2Mbps down and 0.2 up. My wife was getting 80 down and 5 up. I had to figure out what was going on.
My setup includes a slightly older Airport Extreme in the basement that extends the wireless network and which has several machines hardwired to it. I spent a few hours poring over router settings, rebooting devices and even paring the network down to just one router. Nothing was consistently helping.
I did notice a bandwidth difference between the 2.4GHz channel and the 5GHz, but speed tests didn’t yield the same results every time. Oddly, the 2.4GHz channel was generally faster than the 5GHz. I still can’t explain that, but I can tell you what ultimately solved it (after poring over the support forums): toggling Bluetooth.
The instant I turned off Bluetooth on my Air, connection speeds became the same as my wife’s (who never turns on Bluetooth). I know that Bluetooth can conflict with 2.4GHz signals, but it also fixed the 5GHz connection. I have no idea why. Turning it back on again did not change the speeds, it just needed to be toggled off once.
I’ve set up ControlPlane to toggle Bluetooth automatically when connecting and disconnecting my 27” Thunderbolt Display now, which is where I use my external keyboard and mouse. I’m not sure if the problem creeps back up eventually, but I’m just toggling it regularly to avoid the issue altogether. Now that I’ve found it, network connections and speeds are improved across all the networks I connect to.
Hopefully this info is of use to someone else as well.