well 4 41inch and 2 21inch monitor means you must be running them and quite high resolutions...
which means more graphic processing on your video card.
but the issue sounds like youre just overusing your graphic processor and theres lag in even mouse movements. which is not good at all.
having another gpu is always nice, so it can rely on an separate processing unit to provide the visual for you.
and im sure that will drastically increase your performance too.
but spending another $1500 on a gpu, thats up to you lol