SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    366833, 366834, 366835, 366836, 366837, 
    366838
  ) 
  AND feature_id IN (649) 
  AND lang_code = 'en'

Query time 0.00099

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "3.42"
    },
    "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": 6,
      "rows_produced_per_join": 6,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "2.22",
        "eval_cost": "1.20",
        "prefix_cost": "3.42",
        "data_read_per_join": "4K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_features_values`.`feature_id` = 649) and (`webmarco`.`cscart_product_features_values`.`product_id` in (366833,366834,366835,366836,366837,366838)) and (`webmarco`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}