json destructor quite slow

Description

I was running patch over a ~100Mb repodata file and noticed that it was slow due to copying and destroying the entire object multiple times.

I am quite curious if the performance of destroy() could be improved? On my computer, it takes roughly 1 second to parse the file and create the json object, but also 0.5 seconds to destroy the json object.

Reproduction steps

Download a large JSON file, such as https://conda.anaconda.org/conda-forge/linux-64/repodata.json (curl --compressed https://conda.anaconda.org/conda-forge/linux-64/repodata.json)

and run the following code:

#include <iostream>
#include <chrono>
#include <nlohmann/json.hpp>

int
main()
{
    std::ifstream rdata("repodata.json");
    std::unique_ptr<nlohmann::json> j = std::make_unique<nlohmann::json>();
    
    {
        auto t0 = std::chrono::high_resolution_clock::now();
        rdata >> (*j);
        auto t1 = std::chrono::high_resolution_clock::now();
        std::cout << "took " << std::chrono::duration_cast<std::chrono::milliseconds>(t1-t0).count() <<" ms." << std::endl;
    }

    {
        auto t0 = std::chrono::high_resolution_clock::now();
        j.reset();
        auto t1 = std::chrono::high_resolution_clock::now();
        std::cout << "took " << std::chrono::duration_cast<std::chrono::milliseconds>(t1-t0).count() <<" ms." << std::endl;
    }
}

Expected vs. actual results

destruction should have minimal runtime cost

Minimal code example

No response

Error messages

No response

Compiler and operating system

clang 12, macOS

Library version

3.10.5

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

Start with the destructor behavior of nlohmann::json in the header-only nlohmann/json.hpp implementation and reproduce the timing using the large repodata.json example from the issue. Profile destruction and compare the result against parsing, then run the project’s unit tests; done means a validated reduction in destruction time without regressions.