Internet Upload Speed decreases when starting a game

Heyo!

This might not be an OBS specific problem but I need you guys' help in order to be able to stream.

Problem: As soon as I start a game my download speed drops to 10Mbit/s at max, upload is under 1Mbit/s. When checking the upload speed on OBS while streaming it shows around 200Kb/s. This leads to Twitch recognizing my stream being online but being stuck with a frozen picture and loading the video.

Already checked:
- CPU-usage is steadily staying at around 12% while gaming & streaming. --> not the problem
- using an SSD --> not the problem
- RAM-usage is steadily around 50% while gaming & streaming --> not the problem

If you guys need more information I will happily provide. Thank you so much in advance! <3

92.4Mbit/s Downloadspeed
33.1Mbit/s Uploadspeed
while no game is running

7.85 Mbit/s Downloadspeed
0.25 Mbit/s Uploadspeed
while a single player game with no internet connection required is running

Problem solved.
I figured it out! :) Posting my solution in order to hopefully help others:

I'm using a Lenovo PC. Didn't know Lenovo installs the Software "Vantage" and automatically turns on network boost. This will always reserve a huge part of the bandwidth for a game the moment you start one. I basically only entered Lenovo Vantage and turned off the network boost.