Had this windows stuttering problem on league of legends since last June before the recent update as well (I gave up playing before this new update so not sure if it's fixed now). Bought and built a brand new PC with good specs and the network was great. For some reason the system limits the frame rates to 90~144fps, causing significant tearing on the screen whenever the framerate is below 144fps. Afterwards, we temporarily fixed it by choosing "vertical sync" or "ignore vertical sync" in the settings every time I start a new game.