How to throttle bandwith for Xbox SX?

You throttle it at your router

Routers do.have.network management tools

I ended up getting a managed network switch ( I think it's actually a smart switch as most managed switches are hundreds of dollars) to limit the bandwidth usage. I amazed Microsoft doesn't offer a network bandwidth usage limiting feature. Steam has it!

My router/modem doesn't have the ability to throttle bandwidth. I had to buy a separate piece of hardware. From what I can tell many basic/simple routers don't have this feature. I think it's important to point out that if someone wants to throttle with hardware (router/switch) they need to do their due diligence and make sure it has the ability to do so.

Are you downloading to a Xbox console or on a PC?

Sometimes internet providers artificially throttle downloads from high-bandwith sources, but don't actually tell their customers.

Usually for ordinary web surfing, that won't matter.

You could try to start a Speedtest from a well known speedtest site, as those "traffic shaping" Internet providers then release the throttle, so you can't "measure" their bandwith throttling, as long as you use a speedtest.

Check if the downloads continue while you do speedtests.

Be aware, that when the Sea of Thieves game is downloading, you will of course NOT reach your usual bandwith speed.

I'm on comcast and there doesn't seem to be a bandwith CAP per se, but if you use more than a certain amount it starts throttling things like Youtube and bittorrent.