I have the same and it is explainable.
Making the second screen active gives it first priority for the GPU (for example for playing youtube video’s etc.) So what happens is the game becomes GPU limited, which gives the least stutters.
Then you make the main screen active again. The GPU prioritizes the main screen again so the system becomes limited by main thread, which causes stutters.
You can minimize this effect by disabling hardware acceleration in your browser. But then it will always stutter because the system is always limited by main thread. 
Just use your GPU more so it becomes the bottleneck, turn rendering scale up a little.
You want the GPU to be the bottleneck for a stutter free experience.