SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21631, 21632, 21633, 21634, 21635, 21636, 
    21637, 21638, 21639, 21640, 21641, 
    21642, 21643, 21644, 21645, 21646
  )

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_products.product_id in (21631,21632,21633,21634,21635,21636,21637,21638,21639,21640,21641,21642,21643,21644,21645,21646)"
    }
  }
}