SELECT 
  usergroup_ids 
FROM 
  cscart_pages 
  INNER JOIN cscart_ult_objects_sharing ON (
    cscart_ult_objects_sharing.share_object_id = cscart_pages.page_id 
    AND cscart_ult_objects_sharing.share_company_id = 1 
    AND cscart_ult_objects_sharing.share_object_type = 'pages'
  ) 
WHERE 
  page_id = 4

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_pages",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["page_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "cscart_ult_objects_sharing",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "159",
          "used_key_parts": [
            "share_object_id",
            "share_company_id",
            "share_object_type"
          ],
          "ref": ["const", "const", "const"],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      }
    ]
  }
}