tell me why the fuck my game runs better when my browser is open on my other monitor

Magic?

Had that too.
My GPU didn't want to boost the clock with my application.
For me a driver update fixed that.

Is the other monitor plugged in the graphics card, or in the ioshield of the motherboard?

Some laptop? Some have a dedicated gpu and switch between integrated and dedicated depending on the work load. Maybe your browser triggers the gpu and your app does not. In my opinion - get rid of the switching. Only works half the time and is annoying as fuck. Disable integrated in bios and carry on

Watch your gpu clocks with gpu-z or similar