T-mobile 5G Home internet and VPN

Thanks for the update. As you note, I would expect download speeds to decrease with a VPN, but certainly not as much as you're stating. I use Mullvad VPN on my MacBook Pro, and I only see a ~10% decrease in speed.

I'd talk to your IT staff to see what they think. If they can't figure it out, I'd have them call your VPN vendor and ask why this might be happening. T-Mobile would have no reason to prioritize normal Internet traffic over encrypted signals. To them it's just another request.

T-Mobile's Home Internet protocol is called Fixed Wire Internet (FWI), same as Verizon's, which is why you need a gateway as opposed to a cable modem to communicate. The back end of both networks is fiber, so in and of itself, this doesn't explain the slowdown.