Work with two shapes
Try Em and Am. Place each shape separately and check that it sounds clear. Then lift your fingers just above the strings and move them together toward the next shape. Notice which finger can land first.
Count slowly
Count four slow beats per chord and strum only on beat one. Once it works, change every two beats. If a string sounds muffled, slow down and reset; speed only helps when the notes remain clear.
Should I always practise faster?
No. Build relaxed, clean changes first, then gradually increase the tempo.