SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 259698 
  AND lang_code = 'en'

Query time 0.00063

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "514.80"
    },
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "ref",
      "possible_keys": [
        "lang_code",
        "product_id",
        "idx_product_feature_variant_id"
      ],
      "key": "product_id",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 429,
      "rows_produced_per_join": 21,
      "filtered": "4.94",
      "index_condition": "(`webmarco`.`cscart_product_features_values`.`lang_code` = 'en')",
      "cost_info": {
        "read_cost": "429.00",
        "eval_cost": "4.24",
        "prefix_cost": "514.80",
        "data_read_per_join": "16K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "value",
        "value_int",
        "lang_code"
      ]
    }
  }
}