Playing on 1024x768 4:3 stretched is actually killing csgo performance. So i upscaled it to 1152x864 and the change was HUGE.

What is going on? I stopped playing to my 1024x768 res because of terrible, terrible lag and frame stutter.So i upscaled it and WOW. The game was MUCH MORE smoother with almost no fps drops and much much less stutter. Why is this happening?