Overheads, you expect less from yourself, and you're standing in front of NML, a much shorter distance to hit the ball. The ball is also falling from up higher, so you get some extra momentum from that. Very few players hit their overheads nearly as hard as their serves. In your case, your overheads are good enough, but if you served at that speed, your opponent will crush it into a corner.
Once again, your motion on the forward swing is wrong. It's too long after you hit the ball, you never concentrate the rackethead speed anywhere along the swing, instead swinging a long smooth motion, that NEVER cracks the whip.
Think of what it takes to crack a whip to supersonic speeds. You need, at some time, to slow the hand so the head of the whip can speed past, creating the whipping supersonic speed as you pull back. Dont pull back on the serve, but do the rest.