when you cut off device, if the device was activly using internet, all the device packet will go through your PC (the one run netCut). so it appear your own bandwidth reduced. if your use 100% cut off. give it certain time, the device may stop retry send outgoing packet, then the impact will be less. another way is use a dedicated spare PC/root phone to control speed. that you don’t rely on the PC’s own speed.
unfortantuly , there is no solution on this, if you have a space PC / root phone, you can try to run netCut from it, so your game pc bandwidth does not impact.