SELECT 
  cscart_vendor_following.user_id, 
  followed_on, 
  firstname, 
  lastname, 
  email 
FROM 
  cscart_vendor_following 
  LEFT JOIN cscart_users ON cscart_users.user_id = cscart_vendor_following.user_id 
WHERE 
  cscart_vendor_following.vendor_id = 120 
ORDER BY 
  followed_on desc

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "read_sorted_file": {
          "filesort": {
            "sort_key": "cscart_vendor_following.followed_on desc",
            "table": {
              "table_name": "cscart_vendor_following",
              "access_type": "ALL",
              "rows": 8,
              "filtered": 100,
              "attached_condition": "cscart_vendor_following.vendor_id = 120"
            }
          }
        }
      },
      {
        "table": {
          "table_name": "cscart_users",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["user_id"],
          "ref": ["live_clonned.cscart_vendor_following.user_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(cscart_users.user_id = cscart_vendor_following.user_id)"
        }
      }
    ]
  }
}