SELECT 
  * 
FROM 
  cscart_bm_locations as l 
  LEFT JOIN cscart_bm_locations_descriptions as d ON d.location_id = l.location_id 
  AND d.lang_code = 'en' 
WHERE 
  l.layout_id = 1928 
  AND l.dispatch IN ('categories.view') 
  AND (
    FIND_IN_SET(7840, l.object_ids) 
    OR l.object_ids = ''
  ) 
ORDER BY 
  l.object_ids desc 
LIMIT 
  1

Query time 0.04008

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9308.99"
    },
    "ordering_operation": {
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "l",
            "access_type": "ALL",
            "rows_examined_per_scan": 42849,
            "rows_produced_per_join": 428,
            "filtered": "1.00",
            "cost_info": {
              "read_cost": "8709.10",
              "eval_cost": "85.70",
              "prefix_cost": "8794.80",
              "data_read_per_join": "97K"
            },
            "used_columns": [
              "location_id",
              "dispatch",
              "is_default",
              "layout_id",
              "object_ids",
              "custom_html",
              "position"
            ],
            "attached_condition": "((`webmarco`.`l`.`layout_id` = 1928) and (`webmarco`.`l`.`dispatch` = 'categories.view') and (find_in_set(7840,`webmarco`.`l`.`object_ids`) or (`webmarco`.`l`.`object_ids` = '')))"
          }
        },
        {
          "table": {
            "table_name": "d",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "location_id",
              "lang_code"
            ],
            "key_length": "10",
            "ref": [
              "webmarco.l.location_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 428,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "428.49",
              "eval_cost": "85.70",
              "prefix_cost": "9308.99",
              "data_read_per_join": "100K"
            },
            "used_columns": [
              "location_id",
              "lang_code",
              "name",
              "title",
              "meta_description",
              "meta_keywords"
            ],
            "attached_condition": "<if>(is_not_null_compl(d), (`webmarco`.`d`.`location_id` = `webmarco`.`l`.`location_id`), true)"
          }
        }
      ]
    }
  }
}