gameObject movement too slow

Trying to move an object smoothly but all I get is slow movement or or a fast jumps
slow movement:
in Start()
rb.velocity = transform.forward * speed;
jumpy movement:
in FixedUpdate()
rb.velocity = Vector3.MoveTowards(rb.velocity, transform.forward * speed, Time.deltaTime * accelerationSpeed);

using both gives some speed but after start jumpy from current position to destination instead of smooth movement best
can anyone help plz

Sorry to bother you guys my problem was due to an extra rigidbody on a child gameObject.
issue solved