SELECT 
  cscart_product_reviews.product_review_id, 
  cscart_product_reviews.product_id, 
  cscart_product_reviews.user_id, 
  cscart_product_reviews.name, 
  cscart_product_reviews.quick_comment, 
  cscart_product_reviews.comment, 
  cscart_product_reviews.rating_value, 
  cscart_product_reviews.product_review_timestamp 
FROM 
  cscart_product_reviews 
WHERE 
  cscart_product_reviews.product_id IN (40688) 
ORDER BY 
  cscart_product_reviews.product_review_id DESC 
LIMIT 
  2

Query time 0.00076

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "read_sorted_file": {
          "filesort": {
            "sort_key": "cscart_product_reviews.product_review_id desc",
            "table": {
              "table_name": "cscart_product_reviews",
              "access_type": "ref",
              "possible_keys": ["idx_product_id"],
              "key": "idx_product_id",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["const"],
              "rows": 4,
              "filtered": 100,
              "attached_condition": "cscart_product_reviews.product_id <=> 40688"
            }
          }
        }
      }
    ]
  }
}