private Vector3 _prevPosition;
private int _bufferSize = 8;
private int _bufferIndex = 0;
private Vector3[] _buffer;
[SerializeField] private Transform racket;

private void Start()
{
    _buffer = new Vector3[_bufferSize];
    _bufferIndex = 0;
    _prevPosition = racket.transform.position;
}

private void Update()
{
    var currentPosition = racket.transform.position;
    var delta = currentPosition - _prevPosition;
    _buffer[_bufferIndex] = delta;
    _bufferIndex = (_bufferIndex + 1) % _bufferSize;

    var velocity = GetRacketVelocity();
}

private float GetRacketVelocity()
{
    Vector3 total = Vector3.zero;
    for (int i = 0; i < _buffer.Length; i++)
    {
        total += _buffer[i];
    }

    return total.magnitude;
}