Any time I had a student who struggled with their scan on approaches and with "keeping up" I would take them back to basic attitude instrument flying. Go do some slow flight under the hood. This is never an easy thing to do, even for somebody with an instrument rating already. You're trying to balance airspeed, altitude, and power. Then try some turns, climbs, and descents, and maybe even some VOR tracking all while maintaining minimum contrallable airspeed. You don't have a choice but to improve your scan. Practice working on your scan away from the approaches, then go back and try again. You need to tackle the problem, which is not the approach, but rather your scan. Just my technique, I'm sure there will be other good ones.