fastunion vs |= in a loop

I have a std::vector of Roaring64Maps

std::vector<roaring::Roaring64Map> results = // some process

When I use fastunion (probably incorrectly):

    auto sharded_results = new const roaring::Roaring64Map*[results.size()];
    for (size_t i = 0; i < results.size(); ++i) {
        sharded_results[i] = &results.data()[i];
    }

    roaring::Roaring64Map combined = roaring::Roaring64Map::fastunion(results.size(), sharded_results);

It takes almost 5 seconds to combine them. However when I just do:

    roaring::Roaring64Map combined;
      for(const auto& sharded : results) {
        combined |= sharded;
    } 

It takes 2.8 seconds. In this case I only have 4 maps, and the combined carnality ends up just above 3 million.
Am I doing something wrong or is this expected?

Reproduce the comparison using four roaring::Roaring64Map values, the fastunion call, and the loop using operator|=. Start by tracing or benchmarking both entry points in the C++ API; done means explaining whether the timing difference is expected or identifying a reproducible performance problem.