It's amazing how little control we have over our left hand's fingers. The fact is we just don't use them much. Place your left hand, palm down on a flat surface. Curl your fingers so that all four tips of your fingers are pressing down on the surface. Now move your index finger and your middle finger up and down, in an alternating pattern, so that they tap on the surface. Try and do that as fast as possible. That is probably fairly easy for you but if you try the same exercise with your middle finger and your ring finger it becomes more difficult. If you try this exercise with your ring finger and your small finger it becomes extremely difficult.

In order to change chords quickly we need to train our brains to move our left hand's fingers independently. Something we never do. I would recommend practicing the exercise above several times a day. Try it with different combination of fingers and move the fingers forward and back as you tap them. The more you practice gaining control of your left hand's fingers the faster you will be able to learn to change chords.

Best of luck to you and always remember, playing the guitar is a journey with no end.