iPhone 8 personal hotspot extremely slow, even though data coverage is fast and PC WiFi works

(Note, I have already read the KB article at http://support.apple.com/kb/ts2756 , it does not fix it.)

I have observed the following data using speedtest.net:

1) When my PC or my Kindle Fire are connected to my xfinity WiFi router, they get download speeds of about 40 Mbps.

2) When my iPhone is connected to the data network (T-Mobile), it gets download speeds of about 40 Mbps.

3) However, when either my PC or my Kindle Fire are tethered to the iPhone's Personal Hotspot and the iPhone is still on the data network, the PC and the Kindle Fire both get download speeds of about 0.4 Mbps -- that's not a typo, they are about 100x slower than the download speed that I get when using the phone directly.

#1 rules out problems with the WiFi hardware in the PC or the kindle, and #2 rules out problems with the iPhone's data connection. So it seems that there is a bug in the iPhone 8 that is causing the personal hotspot to be throttled to a ridiculously small fraction of the available speed.

I had an iPhone 6 before, and while I do remember that web surfing with the PC tethered to the iPhone tended to be slower than web surfing on the iPhone directly, the difference didn't seem this huge.

Is this just an iPhone bug with no workaround?