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:

   168	view::iterator view::find(stdx::string_view key) const {
   169	    bson_t b;
   170	    bson_iter_t iter;
   171
   172	    if (!bson_init_static(&b, _data, _length)) {
   173	        return end();
   174	    }
   175
   176	    if (bson_iter_init_find(&iter, &b, key.to_string().data())) {
   177	        return iterator(element(iter.raw, iter.len, iter.off));
   178	    }
   179
   180	    return end();
   181	}

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.