Repotting in different mix questions? Miracle Gro?

It has dawned on me that perhaps my plants slooooooow growth is due to them not liking their potting mix or soil. Someone else on here also came up with that same idea, thanks. My plants are three weeks old and almost ready to be transplanted into thier final pots. When should I do that by the way? Should I go with the same mix of 60 percent potting soil, 20 percent Perlite, and 20 percent sand for drainage? The main thing I dont like about my potting soil (Scotts) is all the sticks in it! I wanted a rich black soil with no wood bits. I think my roots may be growing slow because of these obstacles. My other option is that I have a couple bags of Miracle Gro potting mix I could use as the final transfer medium. I have heard good and bad about miracle gro. I think the nutients in that soil may be good as my plants are already taking small nutrients doses in their water. Your thoughts?