SELECT 
  subscription_id 
FROM 
  cscart_product_subscriptions 
WHERE 
  product_id = 300 
  AND user_id = 0

Query time 0.00050

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_subscriptions",
      "access_type": "ref",
      "possible_keys": ["pec", "pd"],
      "key": "pd",
      "key_length": "6",
      "used_key_parts": ["product_id", "user_id"],
      "ref": ["const", "const"],
      "rows": 6,
      "filtered": 100
    }
  }
}