this was my major frustration for a long time ( and still is ) it seemed like i would would never be fast enough but it happens. The only way to get faster is to keep doing it. I know how annoying it is when you want to change chords quickly and you cant do it but the best thing to do is just to repeat it over and over again till you get it. You could also stop strumming while you practice so you can concentrate more. keep practicing ;)