Update query hangs my website when i want to update rows in database

Question

I'm facing an issue in updating rows in mysql table. I'm currently working on social media website. There's a notification table where we've all users notification. For example "John liked your post, Sara commented on your post" Just like this

Table structure is given below.

As you can see i have a column "IsRead". If the user click on "Mark all as read button" a query executes.

UPDATE notification SET isRead = 1 WHERE toUser = $id

When i execute this query it takes about 2 minutes or above and at the end it says connection to server has been lost. Error snap is attached below

Please suggest me what's the right way to create a table for notification. My created table structure is given above.

This is my javascript code:

function markAsRead(){
$('.notifications-area').html('<li><center>Noting to display</center></li>');
$('.list-notification').html('<li><center>Noting to display</center></li>');
$('#countNotifications').html(0);
$('#countNotifications1').html(0);
$.ajax({
        type: 'ajax',
        method:'POST',
        url: "<?php echo site_url()?>/Main/markAsRead",
        datatype:"JSON",
        success:function(responce)
        {   
            
        },
        error:function(e)
        {
           console.log(e);
        }
    });
}

My Controller:

public function markAsRead(){
    
    $data=$this->m->markAsRead();
     echo json_encode($data);
}

Model:

public function markAsRead(){
    $userid = $this->session->userdata('userid');
    $query = $this->db->query("UPDATE notification SET isRead = 1 WHERE toUser = $userid and isRead = 0");
    return $query;
}