Gaming mice have an unfortunate habit of having the marketing department pushing higher and higher polling rates using the logic bigger numbers are better. Unless you are a pro gamer on a very high refresh rate screen you wouldn't notice much difference after probably 250hz or even the default 125hz.
That said we will investigate whats going on, I suspect the absurdly high polling rate is causing excess repaints on the screen and if you have a blurred groupy tab on the screen you may be being rate limited by the blur whilst the GPU is also being used to render that other window.
