Using a T-Mobile sim from my iPhone X in my iPad Mini 6.

The Tmobile phone sim isn't provisioned to be used in a tablet which is why your getting those speeds.

Consulta: https://www.t-mobile.com/support/devices/apple/setting-up-non-t-mobile-apple-devices

Based on this web page, I expected that my iPhone sim would work in the iPad.

Nope. A phone sim will only work in a phone
and a tablet sim will only work in a tablet.

If you go to the tmobile section of the reddit board there's people posting that you can't and if you try you get the speeds in your original post.

Thanks for the update. The Google Fi sim I just got works in either my iPhone phone or iPad but Google Fi doesn't yet support 5G on iPhone/iPad.