SELECT 
  destination_id, 
  rate_value, 
  rate_type 
FROM 
  cscart_tax_rates 
WHERE 
  tax_id = 6 
  AND destination_id = 1

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_tax_rates",
          "access_type": "const",
          "possible_keys": ["tax_rate"],
          "key": "tax_rate",
          "key_length": "6",
          "used_key_parts": ["tax_id", "destination_id"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}