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!