in my cameras (a620 & s3is) speed is about the same, be it in continous mode or custom timer. the custom timer in the end DOES use burst mode, as it only focuses once (when you start the process). disable review time and there you go. and by the way, in bracketing you not only want to avoid lag, but also movement. this means, the custom timer is way better than the normal burst mode, as you dont have to touch the cam or the shutter.
