HWSync hasn't anything to do with hw acceleration, because it's just enabling the vertical-blanc-interrupt-sync. First off this cause the animation to look nice and 2nd it makes the device lags because the software gets sync to the vertical-blanc interrupt of the graphic-processor. This can only work if the software making the transition can draw a full transition in one (1) vertical-blanc (vbl). Obviously this isn't your observeration (lag) and although on many device its cause a reboot loop.