SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    339, 338, 337, 335, 334, 333, 331, 330, 
    329, 341, 342, 343, 371, 372, 373, 374, 
    375, 376, 377, 378
  ) 
  AND feature_id IN (550, 549, 551, 548) 
  AND lang_code = 'en'

Query time 0.00157

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "40.13"
    },
    "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": "PRIMARY",
      "used_key_parts": [
        "feature_id",
        "product_id"
      ],
      "key_length": "6",
      "rows_examined_per_scan": 89,
      "rows_produced_per_join": 88,
      "filtered": "99.75",
      "using_index": true,
      "cost_info": {
        "read_cost": "22.37",
        "eval_cost": "17.76",
        "prefix_cost": "40.13",
        "data_read_per_join": "68K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`pankajecarter_systemfour`.`cscart_product_features_values`.`product_id` in (339,338,337,335,334,333,331,330,329,341,342,343,371,372,373,374,375,376,377,378)) and (`pankajecarter_systemfour`.`cscart_product_features_values`.`feature_id` in (550,549,551,548)) and (`pankajecarter_systemfour`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}