On AT computers you had to go to low frequency to play games built for XT's. For example M1 Abrams tank simulator was a slideshow on a XT at 4.77, slow but almost playable on 9.54, fine on a AT at 12 MHz and unplayable on a 386 at 40 MHz (it was like a movie played on 10x speed). On the 386 with turbo off (16 MHz) was playable.