SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    264208, 264847, 264848, 264849, 264850, 
    264851, 264852, 264853, 264854, 264855
  ) 
  AND feature_id IN (549, 649, 882, 939) 
  AND lang_code = 'en'

Query time 0.00291

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 (264208,264847,264848,264849,264850,264851,264852,264853,264854,264855)) and (`webmarco`.`cscart_product_features_values`.`feature_id` in (549,649,882,939)) and (`webmarco`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id feature_id variant_id
264208 549 1198
264847 549 1198
264848 549 1198
264849 549 1198
264850 549 1198
264851 549 1198
264852 549 1198
264853 549 1198
264854 549 1198
264855 549 1198
264208 649 62612
264847 649 62608
264848 649 62609
264849 649 62610
264850 649 62611
264851 649 62608
264852 649 62609
264853 649 62610
264854 649 62611
264855 649 62612
264208 882 60416
264847 882 60416
264848 882 60416
264849 882 60416
264850 882 60416
264851 882 60412
264852 882 60412
264853 882 60412
264854 882 60412
264855 882 60412
264208 939 63937
264847 939 63937
264848 939 63937
264849 939 63937
264850 939 63937
264851 939 63937
264852 939 63937
264853 939 63937
264854 939 63937
264855 939 63937