Rigidbody2D.addforce giving jerky movevent

I'm attempting to apply a force to the player whilst it is below the maximum speed, then stop applying the force once the max speed is reached, until the player stops holding the relevant button.

Unfortunately, this is causing very jerky behaviour and some framerate drops. The following is being called by FixedUpdate():

 int dir = (int) Input.GetAxisRaw("Strafe");
 
 if(Mathf.Abs(_r.velocity.x) < _vMax && dir != 0)
 {
     _r.AddForce(new Vector2(dir * acceleration, 0), ForceMode2D.Force);
 }

The acceleration is a constant defined in the code, and _r is the player's Rigidbody2D component.