You can get another 56% speedup on the reserve()+emplace_back() version just by replacing (Widget widget) with (const Widget& widget) in the function declaration [0], more of a difference than any of the loops. As well as a further 39% speedup by moving transformed_data into repsonse.data instead of copying it [1]. My takeaway is that C++ makes it extremely easy to copy data around unnecessarily. Also, you should always profile your code for easy wins before making drastic changes.

[0] https://quick-bench.com/q/1Lcfho3hZW2DVgse3xkn82hZsFg

[1] https://quick-bench.com/q/r0DxLEPK68VhBPect3qAhkNIVnA

In this degenerate example, the completely straightforward way is tied for fastest as well. It's clear this has nothing to do with ranges.

    Response response;
    response.data.reserve(widget.data.size());
    std::ranges::transform(widget.data, std::back_inserter(response.data),
        [](const auto& element) {
          return ToData{.title = element.title, .amount = element.amount*42};});
    return response;