A busy connection and growing queues can affect responsiveness even when a separate speed test once showed a high download rate.
Why this happens
A household connection carries more than the programme arriving at the television. Backups, video calls and file transfers can compete for capacity and create delays in the network path. A previous download-speed result does not describe those conditions while an upload is running. The useful clue is timing: does the trouble begin when a particular transfer starts and improve when that transfer is paused? That comparison is more informative than assuming the streaming app suddenly became incompatible.
Try one short comparison with the backup paused, leaving the television, player and source unchanged. If the difference is repeatable, use the router or service’s documented traffic-management options rather than changing random video settings. Queue-management techniques exist specifically to reduce delay under load, but not every router implements the same controls, and no single setting can create bandwidth the connection does not have. Keep other household needs in view before assigning one device priority. A normal working network matters more than chasing the largest speed-test number.
A useful way to work through it
Compare the same source while the large upload is running and paused.
Record whether interruptions track that household activity consistently.
Use only documented router traffic-management controls and preserve their previous settings.