T-Mobile Home Internet throttling speeds to certain servers (10x reduction)

I may be seeing the same thing. My car berate is s bit weaker than yours. I get 2-3 bars of signal. I'm about 3.5 miles from the tower. 
The speeds you get slowed to are the good speeds I get. Normally I get 20 - 30 Mbps. Occasionally I get 50 Mbps. 
Sometimes speeds drop abruptly to < 1 Mbps and sometimes it just stops. I found your comment because I was suspecting that my speed was throttled. The speed change is abrupt. And it is at high use times: evenings and weekends. I don't have a VPN account so I can't repeat your test. 
For a long time I thought it was my network configuration causing the problem. It's a little more complicated than just "plug in and use Wi-Fi" because the gateway is in a building outside of my home where I can get a signal. I use a point-to-point link to connect to my home. Now I'm becoming convince that it's throttling. 
I hope you find a solution and can share it.