SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    12378, 12377, 12376, 12375, 12374, 12373, 
    12372, 12371, 12370, 12369, 12368, 
    12367, 12366, 12365, 12364
  ) 
  AND feature_id IN (624, 625, 553, 627, 628) 
  AND lang_code = 'en'

Query time 0.00659

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "6.21"
    },
    "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",
        "variant_id",
        "lang_code"
      ],
      "key_length": "15",
      "rows_examined_per_scan": 14,
      "rows_produced_per_join": 14,
      "filtered": "100.00",
      "using_index_for_skip_scan": true,
      "cost_info": {
        "read_cost": "4.81",
        "eval_cost": "1.40",
        "prefix_cost": "6.21",
        "data_read_per_join": "10K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`test2_gaseus`.`cscart_product_features_values`.`product_id` in (12378,12377,12376,12375,12374,12373,12372,12371,12370,12369,12368,12367,12366,12365,12364)) and (`test2_gaseus`.`cscart_product_features_values`.`feature_id` in (624,625,553,627,628)) and (`test2_gaseus`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}