Cant seem to switch from D minor to A chord on guitar

1. Why do I struggle with the D chord?
2. Why can't I change chords fast enough?

Why do I struggle with the D chord?

Why is this chord so hard? ... The D chord on guitar is very difficult for beginners because you have to use three 'split' fingers (they're not bunched together in an easy or compact group) and secondly you must avoid playing 2 strings.

Why can't I change chords fast enough?

Try setting the tempo with your right hand, keep it steady and at speed, force your left hand to try and make the changes as best you can. It will get better as you practice making changes instead of playing slowly. And don't forget that there is no law that says every chord strum has to sound good.