Are you using your ISP's modem/router?

Try a bufferbloat test: https://www.waveform.com/tools/bufferbloat

If this is the cause, essentially the ISP is buffering a lot of data on their side to make sure it's ready to send to your router ASAP, but that also means that anything else being requested gets queued behind all of that Steam download data, so it takes forever for anything to load although it should eventually load.

You can also try pinging google or something while downloading, you might see the ping response times go from a few ms to seconds.