SELECT 
  feature_id, 
  feature_type 
FROM 
  cscart_product_features 
WHERE 
  feature_id IN (549, 548, 18, 16)

Query time 0.00005

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_features",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["feature_id"],
      "rows": 4,
      "filtered": 100,
      "index_condition": "cscart_product_features.feature_id in (549,548,18,16)"
    }
  }
}