@Merk1s 

Hi, Thank you very much for the feedback.

"when I try to download a game on Xbox, my internet speed goes up to 70Mbps, then drops to 25Mbps and always stays at this speed."

-Did you spontaneously run a Ookala speedtest to view the Internet Bandwidth and download the game on the Xbox?

Or I have seen some users mentioned when Xbox is downloading a game, it will take the entire bandwidth which makes other devices have a lower bandwidth. I wonder whether it looks like your case.

Wait for your reply and best regards.