It’s a martial cliche but like all cliches, it has roots in truth. When people try to go fast, they often tense up and end up fighting against themselves while trying to execute a technique. When people try and go fast, they often skip steps and bungle the technique.
Trying to go fast often gets people in more trouble than they were before they started – being in a hurry often leads to errors.
Of course we all know that, at some stage, we need to go fast martial arts. My experience has been that when you are first learning a new technique, stay away from going fast. Go slow and think about what you are doing. As things become more natural, then you can push the speed forward.
Make it slow, make it smooth and speed will come soon enough.