SELECT 
  warehouse_id, 
  amount, 
  store_type, 
  position, 
  main_destination_id, 
  pickup_destinations_ids, 
  shipping_destinations_ids, 
  status, 
  wpa.product_id as product_id 
FROM 
  cscart_store_locations AS sl 
  INNER JOIN cscart_warehouses_products_amount AS wpa ON wpa.warehouse_id = sl.store_location_id 
WHERE 
  wpa.product_id IN (221) 
ORDER BY 
  sl.position ASC

Query time 0.00059

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.004181617,
    "filesort": {
      "sort_key": "sl.position",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "wpa",
              "access_type": "ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["const"],
              "loops": 1,
              "rows": 1,
              "cost": 0.00183779,
              "filtered": 100
            }
          },
          {
            "table": {
              "table_name": "sl",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["store_location_id"],
              "ref": ["u428615623_ecartifygonje.wpa.warehouse_id"],
              "loops": 1,
              "rows": 1,
              "cost": 0.00171364,
              "filtered": 100
            }
          }
        ]
      }
    }
  }
}