Apple+ TV app buffering when no others do.

I've seen this question before and the reply is unsatisfactory.

My daughter has the app on her smart TV. We were watching a show, started OK and, in only a few minutes it was buffering. I was troubleshooting and no other app, Disney+, for instance, buffered in the least.

I checked the WiFi at her TV and it's around 650MBPS. That is lightening fast for any streaming app.

I use a Firestick and an nVidia Shield for my Apple+ app and never have buffering.

I'm confused.

Of course I suggested she get a Firestick.