Mouse movement lagging

Remove the Time.deltaTime. This is because of the Input.GetAxis for the mouse inputs gives you the pixels the mouse moved since the last frame. This means on longer frames the input value you get from Input.GetAxis for your mouse will be higher and on slower frames, the input will be slower. So all you would have to do to fix this would be this:

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

public class cameramovescript : MonoBehaviour
{

    public float mouseSensitivity = 100f;
    public Transform playerBody;

    float xRotation = 0f;

    // Start is called before the first frame update
    void Start()
    {
        Cursor.lockState = CursorLockMode.Locked;
    }

    // Update is called once per frame
    void Update()
    {
        float mouseX = Input.GetAxis("Mouse X") * mouseSensitivity * Time.deltaTime;
        float mouseY = Input.GetAxis("Mouse Y") * mouseSensitivity * Time.deltaTime;

        xRotation -= mouseY;
        xRotation = Mathf.Clamp(xRotation, -90f, 90f);

        transform.localRotation = Quaternion.Euler(xRotation, 0f, 0f);
        playerBody.Rotate(Vector3.up * mouseX);
    }
}

Here is the function I like to use in my projects:

    float GetMouseOrStickLookAxis(string mouseInputName, string stickInputName)
    {
        if (CanProcessInput())
        {
            // Check if this look input is coming from the mouse
            bool isGamepad = Input.GetAxis(stickInputName) != 0f;
            float i = isGamepad ? Input.GetAxis(stickInputName) : Input.GetAxisRaw(mouseInputName);

            // handle inverting vertical input
            if (invertYAxis)
                i *= -1f;

            // apply sensitivity multiplier
            i *= lookSensitivity;

            if (isGamepad)
            {
                // since mouse input is already deltaTime-dependant, only scale input with frame time if it's coming from sticks
                i *= Time.deltaTime;
            }
            else
            {
                // reduce mouse input amount to be equivalent to stick movement
                i *= 0.01f;
#if UNITY_WEBGL
                // Mouse tends to be even more sensitive in WebGL due to mouse acceleration, so reduce it even more
                i *= webglLookSensitivityMultiplier;
#endif
            }

            return i;
        }

        return 0f;
    }

If you need a more in depth answer check out this article: Mouse movement with deltaTime

Have a great day!