SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (727, 728, 729, 730, 731) 
  AND feature_id IN (50, 51) 
  AND lang_code = 'en'

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00787146,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features_values",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "fl",
          "key_length": "9",
          "used_key_parts": ["feature_id", "lang_code"],
          "loops": 1,
          "rows": 20,
          "cost": 0.00787146,
          "filtered": 50,
          "attached_condition": "cscart_product_features_values.product_id in (727,728,729,730,731) and cscart_product_features_values.feature_id in (50,51) and cscart_product_features_values.lang_code = 'en'",
          "using_index": true
        }
      }
    ]
  }
}