… it might be because your iPhone is plugged in and “Personal Hotspot” is enabled.
I’ve been struggling with this one for a few days now — I have great broadband connection in my office but the internet on my desktop computer was super slow. Then I realized it was only happening when my iPhone was plugged in for charging. Most obviously, any site using Youtube wouldn’t fully load. Youtube itself wouldn’t load.
And then I realized I had “Personal Hotspot” activated on my iPhone. (I use the Personal Hotspot setting when out of the office to get an internet connection into my laptop — and sometimes forget to turn it off.)
I deactivated “Personal Hotspot” and internet speed on the desktop returned to normal.
Either the computer got confused with multiple choices for internet access and just bogged down. Or, stupidly, the computer was using the iPhone connection via USB rather than the much better office broadband connection.
Just sharing on the off chance it help someone else!