Unity Simple Movement

SimpleMovement.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SimpleMovement : MonoBehaviour
{
    public float mainSpeed = 100.0f; //regular speed
    float shiftAdd = 250.0f; //multiplied by how long shift is held.  Basically running
    float maxShift = 1000.0f; //Maximum speed when holdin gshift
    float camSens = 0.25f; //How sensitive it with mouse
    private Vector3 lastMouse = new Vector3(255, 255, 255); //kind of in the middle of the screen, rather than at the top (play)
    private float totalRun = 1.0f;

    void Update()
    {
        lastMouse = Input.mousePosition - lastMouse;
        lastMouse = new Vector3(-lastMouse.y * camSens, lastMouse.x * camSens, 0);
        lastMouse = new Vector3(transform.eulerAngles.x + lastMouse.x, transform.eulerAngles.y + lastMouse.y, 0);
        if (Input.GetKey(KeyCode.Mouse1))
        {
            transform.eulerAngles = lastMouse;
        }
        lastMouse = Input.mousePosition;
        //Mouse  camera angle done.  

        //Keyboard commands
        float f = 0.0f;
        Vector3 p = GetBaseInput();
        if (Input.GetKey(KeyCode.LeftShift))
        {
            totalRun += Time.deltaTime;
            p = p * totalRun * shiftAdd;
            p.x = Mathf.Clamp(p.x, -maxShift, maxShift);
            p.y = Mathf.Clamp(p.y, -maxShift, maxShift);
            p.z = Mathf.Clamp(p.z, -maxShift, maxShift);
        }
        else
        {
            totalRun = Mathf.Clamp(totalRun * 0.5f, 1f, 1000f);
            p = p * mainSpeed;
        }

        p = p * Time.deltaTime;
        Vector3 newPosition = transform.position;
        if (Input.GetKey(KeyCode.Space))
        { //If player wants to move on X and Z axis only
            transform.Translate(p);
            newPosition.x = transform.position.x;
            newPosition.z = transform.position.z;
            transform.position = newPosition;
        }
        else
        {
            transform.Translate(p);
        }

    }

    private Vector3 GetBaseInput()
    { //returns the basic values, if it's 0 than it's not active.
        Vector3 p_Velocity = new Vector3();
        if (Input.GetKey(KeyCode.W))
        {
            p_Velocity += new Vector3(0, 0, 1);
        }
        if (Input.GetKey(KeyCode.S))
        {
            p_Velocity += new Vector3(0, 0, -1);
        }
        if (Input.GetKey(KeyCode.A))
        {
            p_Velocity += new Vector3(-1, 0, 0);
        }
        if (Input.GetKey(KeyCode.D))
        {
            p_Velocity += new Vector3(1, 0, 0);
        }
        return p_Velocity;
    }
}