SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    66749, 66748, 66743, 66747, 66756, 66746, 
    66745, 66744, 66742, 66775, 66776, 
    66777, 66778, 66779, 66780, 66781, 
    66740, 66739, 66738, 66737, 66736, 
    66735, 66734, 66733, 66732, 66731, 
    66730, 66729, 66741, 66728, 66773, 
    66770, 66769, 66771, 66768, 66767, 
    66766, 66765, 66763, 66762, 66764, 
    66761, 66760, 66772, 66727, 66757, 
    66759, 66785, 66784, 66774, 66758, 
    66786, 66755, 66754, 66753, 66752, 
    66751, 66750, 66783, 66782
  ) 
  AND feature_id IN (63, 104, 67) 
  AND lang_code = 'en'

Query time 0.00764

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "37.00"
    },
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "lang_code",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id",
        "fl"
      ],
      "key": "idx_product_feature_variant_id",
      "used_key_parts": [
        "product_id",
        "feature_id",
        "lang_code"
      ],
      "key_length": "12",
      "rows_examined_per_scan": 180,
      "rows_produced_per_join": 180,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "19.00",
        "eval_cost": "18.00",
        "prefix_cost": "37.00",
        "data_read_per_join": "7K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`cscart_db`.`cscart_product_features_values`.`product_id` in (66749,66748,66743,66747,66756,66746,66745,66744,66742,66775,66776,66777,66778,66779,66780,66781,66740,66739,66738,66737,66736,66735,66734,66733,66732,66731,66730,66729,66741,66728,66773,66770,66769,66771,66768,66767,66766,66765,66763,66762,66764,66761,66760,66772,66727,66757,66759,66785,66784,66774,66758,66786,66755,66754,66753,66752,66751,66750,66783,66782)) and (`cscart_db`.`cscart_product_features_values`.`feature_id` in (63,104,67)) and (`cscart_db`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}