Tetris Game Blocks falling too fast

I've created a simple Tetris game that is working for the most part but my blocks are falling too fast. I think it has something to do with using Time.time but I'm a beginner and am unsure how to proceed. Can you spot the error in my scripts below?

Spawner.cs using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class Spawner : MonoBehaviour {
 
     public GameObject[] groups;
 
     public void spawnNext(){
         int i = Random.Range (0, groups.Length);
         //Spawn group at current position
         Instantiate(groups[i], transform.position, Quaternion.identity);
     }
 
     void Start() {
         spawnNext ();
     }
 }

Grid.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Grid : MonoBehaviour {
 
     public static int w = 10;
     public static int h = 20;
     public static Transform[,] grid = new Transform [w,h];
 
     public static Vector2 roundVec2(Vector2 v) {
         return new Vector2 (Mathf.Round (v.x), Mathf.Round (v.y));
     }
 
     public static bool insideBorder(Vector2 pos) {
         return ((int)pos.x >= 0 && (int)pos.x < w && (int)pos.y >= 0);
     }
 
     public static void deleteRow(int y) {
         for (int x = 0; x < w; ++x) {
             Destroy (grid [x, y].gameObject);
             grid [x, y] = null;
         }
     }
 
     public static void decreaseRow(int y) {
         for (int x = 0; x < w; ++x) {
             if (grid [x, y] != null) {
                 //move one towards bottom
                 grid[x,y-1] = grid[x,y];
                 grid [x, y] = null;
                 //update block position
                 grid[x, y-1].position += new Vector3(0,-1,0);
             }
         }
     }
 
     public static bool isRowFull(int y) {
         for (int x = 0; x < w; ++x)
             if (grid[x, y] == null)
                 return false;
         return true;
     }
 
     public static void decreaseRowsAbove(int y) {
         for (int i = y; i < h; ++i)
             decreaseRow (i);
     }
 
     public static void deleteFullRows() {
         for (int y = 0; y < h; ++y) {
             if (isRowFull(y)) {
                 deleteRow(y);
                 decreaseRowsAbove(y+1);
                 --y;
             }
         }
     }
 
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
         
     }
 }

Group.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Group : MonoBehaviour {
 
 
 
     bool isValidGridPos() {        
         foreach (Transform child in transform) {
             Vector2 v = Grid.roundVec2(child.position);
 
             // Not inside Border?
             if (!Grid.insideBorder(v))
                 return false;
 
             // Block in grid cell (and not part of same group)?
             if (Grid.grid[(int)v.x, (int)v.y] != null &&
                 Grid.grid[(int)v.x, (int)v.y].parent != transform)
                 return false;
         }
         return true;
     }
 
     void updateGrid(){
         for (int y = 0; y < Grid.h; ++y)
             for (int x = 0; x < Grid.w; ++x)
                 if (Grid.grid [x, y] != null)
                     if (Grid.grid [x, y].parent == transform)
                         Grid.grid [x, y] = null;
         foreach (Transform child in transform) {
             Vector2 v = Grid.roundVec2 (child.position);
             Grid.grid [(int)v.x, (int)v.y] = child;
         }
     }
 
     // Use this for initialization
     void Start () {
         if (!isValidGridPos ()) {
             Debug.Log ("GAME OVER");
             Destroy(gameObject);
         }
         
     }
     
     // Update is called once per frame
     void Update() {
 
         float lastFall = 0;
         // Move Left
         if (Input.GetKeyDown(KeyCode.LeftArrow)) {
             // Modify position
             transform.position += new Vector3(-1, 0, 0);
 
             // See if valid
             if (isValidGridPos())
                 // It's valid. Update grid.
                 updateGrid();
             else
                 // It's not valid. revert.
                 transform.position += new Vector3(1, 0, 0);
         }
 
         // Move Right
         else if (Input.GetKeyDown(KeyCode.RightArrow)) {
             // Modify position
             transform.position += new Vector3(1, 0, 0);
 
             // See if valid
             if (isValidGridPos())
                 // It's valid. Update grid.
                 updateGrid();
             else
                 // It's not valid. revert.
                 transform.position += new Vector3(-1, 0, 0);
         }
 
         // Rotate
         else if (Input.GetKeyDown(KeyCode.UpArrow)) {
             transform.Rotate(0, 0, -90);
 
             // See if valid
             if (isValidGridPos())
                 // It's valid. Update grid.
                 updateGrid();
             else
                 // It's not valid. revert.
                 transform.Rotate(0, 0, 90);
         }
 
         // Move Downwards and Fall
         else if (Input.GetKeyDown(KeyCode.DownArrow) ||
             Time.time - lastFall >= 1) {
             // Modify position
             transform.position += new Vector3(0, -1, 0);
 
             // See if valid
             if (isValidGridPos()) {
                 // It's valid. Update grid.
                 updateGrid();
             } else {
                 // It's not valid. revert.
                 transform.position += new Vector3(0, 1, 0);
 
                 // Clear filled horizontal lines
                 Grid.deleteFullRows();
 
                 // Spawn next Group
                 FindObjectOfType<Spawner>().spawnNext();
 
                 // Disable script
                 enabled = false;
             }
 
             lastFall = Time.time;
         }
     }
 }