SELECT 
  product_id, 
  is_stock_split_by_warehouses 
FROM 
  cscart_products 
WHERE 
  product_id IN (91, 96, 97, 98)

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00774628,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_products",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "loops": 1,
          "rows": 4,
          "cost": 0.00774628,
          "filtered": 100,
          "attached_condition": "cscart_products.product_id in (91,96,97,98)"
        }
      }
    ]
  }
}