SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    431561, 431562, 431563, 431564, 431565, 
    431566, 431567, 431568, 431569, 431587, 
    431588, 431589, 431590, 431591, 431592, 
    431593, 431594, 431595, 431596, 431597, 
    431598, 431599, 431600, 431601, 431602, 
    431603, 431604
  ) 
  AND feature_id IN (626, 860) 
  AND lang_code = 'en'

Query time 0.00408

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "22.74"
    },
    "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": 54,
      "rows_produced_per_join": 54,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "11.95",
        "eval_cost": "10.80",
        "prefix_cost": "22.75",
        "data_read_per_join": "41K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_features_values`.`product_id` in (431561,431562,431563,431564,431565,431566,431567,431568,431569,431587,431588,431589,431590,431591,431592,431593,431594,431595,431596,431597,431598,431599,431600,431601,431602,431603,431604)) and (`webmarco`.`cscart_product_features_values`.`feature_id` in (626,860)) and (`webmarco`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}