how to slow down internet?

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There are programs like NetLimiter you can install on her computer, but she can probably get around them rather easily. You can run QoS on your router if it supports it and set her traffic to a low priority, but if nobody else is using it she would still get the full speed.

Any solution you end up with is either going to get bypassed or just make her angry at you. Since it sounds like the problem is mainly the noise, you might be able to get your parents to step in if you want to, or the best solution might be to take apart the walls and add soundproof insulation, or move to a different room, or use something like headphones with white noise to block it out.

Technology can't cure assholes, just look what happened when social media became a widespread thing.