You don’t control a movie, those two aren’t comparable

Are you downloading to an sd card? If so, that’s the issue, steam downloads to ram and then has to wait for the sd card to write the already downloaded data.

My next idea would be to try connecting to the internet using a usb c ethernet adapter of some kind. Is this only the steam deck, or are other steam clients on the same network affected too?

ebay has cheap intel gigabit nics which i havent had issues with