Creating a new network location also does nothing for me. I suspect any of the proposed fixes only appear to work because, by luck, the wi-fi behaved itself for a while after making the change. It looks like Apple has simply failed us on this one.
Turning off "limit IP address tracking" does nothing either. Doesn't matter which browser I use; the download speeds are awful at some random time after a day or so since reboot.
And it's nothing to do with wi-fi signal strength; once the problem arises, I can go and sit right next to the router, reset the router, and still have the same pathetic speeds.
As with all other users complaining about this issue, no other devices are having problems (not even my lousy phone, which has trouble with other routers).