I’ve experienced this on two different laptops now: the wireless connections start to suck. Out of nowhere the network just slows to a crawl. Plug into ethernet and you’re jake. All other wireless devices on the network are humming away just fine with no loss of bandwidth. It’s just the Apple laptops. It’s infuriating, but there’s a very simple (albeit weird) fix for it.
Just go to network preferences and pull down the location menu. From there select edit locations. Create a new location. Doesn’t matter what you call it, it’s all arbitrary. Then delete whatever location you were using, even if it is “Automatic”. Switch to the new location and set it up like normal.
God only knows why this works, but it does.