This may be a little long but hopefully worth it for those of you using USB C hubs.
I had the same issue with my new 2021 MacBook Pro M1 Pro (and posted about it previously). It was intermittent, though. Ethernet still worked like a charm. They were very nice at Apple support, but they could not figure out the source after a bunch of calls. Because it was within my 90-day return period at Costco, I returned it and bought another one, figuring it was a bad wifi antenna. Now, the same thing is happening with the new one, but mainly right after it comes out of sleep (which is different behavior). I "usually" can resolve it by toggling my wifi on/off or restarting the machine. (So, I'm feeling a little guilty right now, which you'll know why after reading below because I otherwise loved that machine ... and I have been a Windows guy for the longest while.)
This morning after waking my machine from sleep, the same thing happened. Coincidentally, I was shopping yesterday on Amazon for a USB C hub for my daughter's MacBook Air and read one review that complained about the hub interfering with wifi connectivity. On a hunch just now, I unplugged my own USB C hub, and voila, my Speedtest.net download speeds returned to normal (e.g., 350/380 Mbps). I plugged it back in, and everything gradually slows down (eventually dropping to 1 Mbps to 25 Mbps). I repeated this process multiple times just now with the same consistent response.
My current setup is a USB C hub plugged into the USB C port on the right side of my machine. Connected to that hub is a powered USB-A hub. (I tried just now stretching the cords to their full lengths to create some separation, with no change.) Between the two hubs, I have connected the following devices: HDMI monitor, wired Logitech keyboard, wired Logitech mouse, Logitech HD 1080p webcam, Envato Wave 3 microphone (which is very new), and a Wacom tablet (just connected the other day for first time). I am using the HDMI port on the right side of the machine (so I can have dual monitors). On the left side, I have an SSD connected via USB C (also recently connected), and the audio jack connected to external speakers.
Things are sorta better now on their own, which typically happens, but not great. On a side note, once the problem occurs, Safari has a much harder time bouncing back than Chrome does, at least this morning it does. However, I just tried unchecking "Limit IP Address Tracking" as suggested above, and Safari is doing better than it was - now 185 Mbps on average versus Chrome's average of 220 Mbps. Neither of these speeds is as good as my top speeds without the hub plugged in.
On another note, I have no bluetooth devices connected to my machine (or hub). I disabled bluetooth anyway, but it doesn't seem to improve things back to the 350/380 Mbps range.
I will continue to diagnose and see if I can pinpoint the actual device causing the problem (which is probably the USB C hub). I may order a bunch of different hubs, but if you want one with a lot of ports, they're all third-party. I shelled out $69 yesterday to buy Apple's own USB-C Digital AV Multiport Adapter, on top of another third-party one for my daughter, but that's only because it's for her. I typically am too cheap when buying things for myself (my MacBook being the sole exception ... my previous Windows laptop was $500), but I may now consider doing the same for myself. If I learn anything new, I will report here.
Sorry this is so long.
