when the lens is mounted on the camera, it will be switched to maximum aperture to maintain a viewfinder as bright as possible. this also helps the AF points to detect contrast n focus more accurately. so from this theory alone, bigger aperture = more light enters thru the lens, n u will get a brighter viewfinder + the AF points receive more light for focusing. since there's more light available, it will be easier for the AF points to detect the contrast n focus accurately. this is particularly useful when tryin to focus in low light situations.
otherwise, smaller max aperture = less light enters the lens = dimmer viewfinder and the AF system will hunt more than usual in dark situations.