The current version of bsoncxx::document::view::find() is implemented in terms of bson_iter_init_find(). This implementation requires a call to string_view::to_string() (which performs an allocation), in order to conform to the bson_iter_t API (which expects null-terminated strings). See snippet below:
This method could be rewritten to avoid the allocation by manually iterating the bson_iter_t to find the desired element, instead of relying on bson_iter_init_find().
Other calls to string_view::to_string() could possibly be audited to determine whether similar performance improvements could be gained elsewhere.