Don't make using the down smash a habit because of the lag after the move. What you will learn with experience is in smash, you need to frame your attacks as much as possible around when your opponent creates a vulnerability. For instance, if you are marth and you fsmash and hit nothing but air, you are vulnerable. If your opponent does not know how to l-cancel (even if they do sometimes), they are vulnerable when landing on the ground, particularly after an aerial attack (that misses). After a roll, your opponent is stuck there and can't do any sort of attack for a fraction of a second. There is also a small portion of a roll where your opponent is vulnerable to your attack, similar to how your opponent who is over 100% and grabbing on the ledge becomes vulnerable for a brief period when rolling back up on stage. Timing your attacks to take advantage of these vulnerabilities is a large part of getting good. When you watch a video of pros playing, it looks simple to you but it is in fact quite complex. There are many aspects of their games that you simply do not comprehend yet.