SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    431542, 431543, 431544, 431545, 431546, 
    431547, 431548, 431549, 431550, 431551, 
    431677, 431678, 431679, 431680, 431681, 
    431682, 431683, 431684, 431685, 431686
  ) 
  AND feature_id IN (626, 860) 
  AND lang_code = 'en'

Query time 0.00203

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "17.11"
    },
    "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": "lang_code",
      "used_key_parts": [
        "lang_code",
        "feature_id",
        "product_id"
      ],
      "key_length": "12",
      "rows_examined_per_scan": 40,
      "rows_produced_per_join": 40,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "9.11",
        "eval_cost": "8.00",
        "prefix_cost": "17.11",
        "data_read_per_join": "30K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_features_values`.`product_id` in (431542,431543,431544,431545,431546,431547,431548,431549,431550,431551,431677,431678,431679,431680,431681,431682,431683,431684,431685,431686)) and (`webmarco`.`cscart_product_features_values`.`feature_id` in (626,860)) and (`webmarco`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}