Consistently slow and unreliable Wi-Fi everywhere I go. 2012 MBP
Since I wrote this post my Internet problems seem to have gotten a lot better. As far as I can remember, I only made two more changes. I can't say for sure which one of these helped, or if either them helped at all and maybe there was some other factor I'm forgetting, but they may be worth a try:
I changed my DNS settings to use Google's Public DNS. I can't remember what they were previously, but I definitely remember playing around with my DNS settings a few months ago so this may have been the culprit.
I changed the automatic MU settings, following the instructions on this page.
Hopefully someone will find one or both of these to be helpful!
After lots of fumbling I discovered that the wifi speed is inversely proportional to the CPU temperatures. I installed TG Pro and found that, without altering the internal fan speed or using external cooling, the CPU's would approach 200 F after about 30 min of on time. With TG Pro I was able to increase the internal fan speed (from the built in speed of 1299 rpm to 3392 rpm) and that reduced the CPU temps to 102 F and the wireless speed was my "normal" 75 mbps.