Unity smooth following 2D camera not working properly on different resolutions

I´m working in a third person 2D game with oblique camera view in Unity and I want the camera to follow the character smoothly. I´m using Vector3.lerp and works like a charm in mostly all the resolutions but when I change to 1080x720 or 600x480 (for example) it doesn´t work well.

I´m using an othographic camera which orthographic size changes depending on the resolution for pixel perfect. When I deactivate the pixelPerfect script doesn´t work in any of the resolutions. I post the pixelPerfect script:

 [System.Serializable]
 public class PerfectOverride
 {
     public int referenceOrthographicSize;
     public float referencePixelsPerUnit;
 }
 
 public class PerfectPixel : MonoBehaviour
 {
     public int referenceOrthographicSize;
     public float referencePixelsPerUnit;
     public List<PerfectOverride> overrides;
 
     private int lastSize = 0;
 
     // Use this for initialization
     void Start()
     {
         UpdateOrthoSize();
     }
 
     PerfectOverride FindOverride(int size)
     {
         return overrides.FirstOrDefault(x => x.referenceOrthographicSize == size);
     }
 
     void UpdateOrthoSize()
     {
         lastSize = Screen.height;
 
         // first find the reference orthoSize
         float refOrthoSize = (referenceOrthographicSize / referencePixelsPerUnit) * 0.5f;
 
         // then find the current orthoSize
         var overRide = FindOverride(lastSize);
         float ppu = overRide != null ? overRide.referencePixelsPerUnit : referencePixelsPerUnit;
         float orthoSize = (lastSize / ppu) * 0.5f;
 
         // the multiplier is to make sure the orthoSize is as close to the reference as possible
         float multiplier = Mathf.Max(1, Mathf.Round(orthoSize / refOrthoSize));
 
         // then we rescale the orthoSize by the multipler
         orthoSize /= multiplier;
 
         // set it
         this.GetComponent<Camera>().orthographicSize = orthoSize;
 
         Debug.Log("Last Screen height "+lastSize + " Ortho Size" + orthoSize + " Multiplier " + multiplier + " PPU " + ppu);
     }
 
     // Update is called once per frame
     void Update()
     {
         #if UNITY_EDITOR
             if (lastSize != Screen.height)
             {
                 UpdateOrthoSize();
                 GetComponent<CameraCenterController>().CameraChanged = true;
             }
         #endif
     }
 }

I must say that, when I use a fixed value as the interpolate in Vector3.lerp, instead Time.Deltatime, it works fine (but less smooth) in all resolutions but still have problems with 1080x720 and 600x480 working only in the Y axis and not in the X.

I paste my code for understanding:

  public float speed;
  public Vector3 distance;

 private void FixedUpdate()
 {
         PlayerCentered();
 }
 
 void PlayerCentered()
 {
         Vector3 positionDesired = player.transform.position + distance;
         Vector3 smoothPosition= Vector3.Lerp(transform.position, positionDesired, speed /*Time.Deltatime*/);
         transform.position = smoothPosition;
 }

I would like to know if you guys know why this is happening and if there is a solution. I research about this problem and everybody talks about lerp and smoothDamp but nobody seems to have problems when changing the resolution.

Thanx!