How to know how fast an object is moving?

Unity Engine
,
1

In my game you can slice things in half, but I want to make sure the player is actually moving his sword fast in order to slice things.

        speed = Vector3.Distance(transform.position, lastPos) / Time.deltaTime;

        lastPos = transform.position;

This code works for getting the speed of the sword, but only if the player is standing still, the moment the player starts moving with the sword the speed jumps up to over 500. Why does that happen?
Full code

using UnityEngine;

public class SliceListener : MonoBehaviour
{
    public Slicer slicer;

    private float speed;
    private Vector3 lastPos;

    private void Start()
    {
        lastPos = transform.position;
    }

    private void OnTriggerEnter(Collider other)
    {
        Explosive explosive = other.GetComponent<Explosive>();

        if ( explosive && speed > 2 / Time.timeScale)
        {
            explosive.Explode();

            return;
        }

        if ( speed < 4 / Time.timeScale && !other.CompareTag("Bullet") )
        {
            return;
        }
       
        slicer.isTouched = true;

        Enemy enemy = other.GetComponent<Enemy>();

        if ( enemy )
        {
            Destroy(enemy);

            Destroy(enemy.GetComponentInChildren<AudioSource>());
        }

        Turret turret = other.GetComponent<Turret>();

        if ( turret )
        {
            Destroy(turret);

            Destroy(turret.GetComponentInChildren<AudioSource>());
        }
    }

    private void Update()
    {
        speed = Vector3.Distance(transform.position, lastPos) / Time.deltaTime;

        lastPos = transform.position;
    }
}
2

transform.position will return the world-space position. When your player moves so does the sword (player speed + sword speed). If your sword is parented to your player you may use transform.localPosition to calculate movement within your player’s local-space.

3

Tried that when the problem first came up and it didn’t work at all, but after rethinking it I realized I was using the swords local position and not the hand holding the swords local position. Now it works. Thanks.