help needed: moving smoothly in 4 directions + 1 constant forward acceleration ?

hey everyone! i need some help with a simple moving script problem. Just before we go into details, I'm rookie, so detailed "instructions" are welcome

what i want to acchive:

a spaceship which can fly into 4 directions ( up, down, left, right and for diagonally) and a constant "forward" acceleration. it a firstperson view game. Maybe the movement from starfox is an good example, basically an on-rail shooter with this movement.

right now it's looking like this:

1 gameobject,

our shipour ship forward acceleration (js):

 ar moveSpeed = 10.0;
 var sideMovement = 10.0;
 var upMovement = 10.0;
 function Update ()
 {
 var sideMovement = Input.GetAxis("Horizontal") * moveSpeed;
  
 sideMovement *= Time.deltaTime;
  
 transform.Translate(sideMovement,0,moveSpeed);
 
 var upMovement = Input.GetAxis("Vertical") * moveSpeed;
  
 upMovement *= Time.deltaTime;
  
 transform.Translate(upMovement,0,moveSpeed);
 }  

our ship movement:

 var keyboardSpeed = 20.0;
  function FixedUpdate () {
     // This is where we move the object.
  
     // Get input from the keyboard, with automatic smoothing (GetAxis instead of GetAxisRaw).
     // We always want the movement to be framerate independent, so we multiply by Time.deltaTime.
     var keyboardX = Input.GetAxis("Horizontal") * keyboardSpeed * Time.deltaTime;
     var keyboardY = Input.GetAxis("Vertical") * keyboardSpeed * Time.deltaTime;
  
     // Calculate the new position based on the above input.
     // If you want to limit the movement, you can use Mathf.Clamp
     // to limit the allowed range of newPos.x or newPos.y.
     var newPos = rigidbody.position + Vector3(keyboardX, keyboardY, 0.0);
  
     // Move the object.
     rigidbody.MovePosition(newPos);
 }
  
 // Require a Rigidbody component to be attached to the same GameObject.
 @script RequireComponent(Rigidbody) 

THE PROBLEM:

if we move our ship in any direction (especially in the diagonal ones) the moving animation looks kinda laggy or "not smooth"). how can we solve this? any script snips & pieces are welcome

EDIT: this was just our first way to do it, i have no problem with replacing the code with an other one!