SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    431754, 431755, 431756, 431757, 431758, 
    431759, 431760, 431761, 431762, 431763, 
    431764, 431765, 431766
  ) 
  AND feature_id IN (626, 860) 
  AND lang_code = 'en'

Query time 0.00147

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "11.47"
    },
    "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": 26,
      "rows_produced_per_join": 26,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "6.27",
        "eval_cost": "5.20",
        "prefix_cost": "11.47",
        "data_read_per_join": "20K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_features_values`.`product_id` in (431754,431755,431756,431757,431758,431759,431760,431761,431762,431763,431764,431765,431766)) and (`webmarco`.`cscart_product_features_values`.`feature_id` in (626,860)) and (`webmarco`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id feature_id variant_id
431754 626 67587
431755 626 7159
431756 626 7168
431757 626 7169
431758 626 7170
431759 626 7160
431760 626 7161
431761 626 7162
431762 626 7163
431763 626 7164
431764 626 7165
431765 626 7166
431766 626 7167
431754 860 64236
431755 860 64236
431756 860 64236
431757 860 64236
431758 860 64236
431759 860 64236
431760 860 64236
431761 860 64236
431762 860 64236
431763 860 64236
431764 860 64236
431765 860 64236
431766 860 64236