SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    339, 1565, 1566, 1567, 1568, 1569, 1570, 
    5556
  ) 
  AND feature_id IN (69) 
  AND lang_code = 'en'

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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",
      "key_length": "6",
      "used_key_parts": ["feature_id", "product_id"],
      "rows": 8,
      "filtered": 75,
      "attached_condition": "cscart_product_features_values.feature_id = 69 and cscart_product_features_values.product_id in (339,1565,1566,1567,1568,1569,1570,5556) and cscart_product_features_values.lang_code = 'en'",
      "using_index": true
    }
  }
}