Using Lerp in a Constant Moving Object

Hi there Folks, I really need a little help in that part of the code, I'm with an object that is constantly moving in position Z forward, and I need to move it in position X without it losing speed. But what happens is exactly that, when he uses Lerp, it loses speed for a few thousandths of seconds and ends up creating a strange and really annoying effect....

private IEnumerator Move(int whereToMove)// 0 Up ; 1 Down- ; 2 Left ; 3 Right { switch (whereToMove) { case 0:

             break;

         case 1:
       
             break;
         case 2:
                            isMoving = true;
             runTime = 0;
             startSlimePosition = transform.position;
             endSlimePosition = new Vector3
                 (startSlimePosition.x - 2.80f, transform.position.y, transform.position.z);

             while (runTime < runDuration)
             {
                 runTime += Time.deltaTime;
                 transform.position = Vector3.Lerp
                     (startSlimePosition, endSlimePosition, runTime / runDuration);
                 yield return null;
             }

             isMoving = false;
             break;