C++ rapidjosn SetObject took long time to finish

I have a class that contains simple attribute and I want to convert it to json. The method to generate json is below:

std::string Message::GetIncomingMessage(){
        rapidjson::Document d; // Null
        d.SetObject();
        rapidjson::Document::AllocatorType& allocator = d.GetAllocator();
        
        rapidjson::Value from;
        from.SetString(rapidjson::StringRef(this->from.c_str(), this->from.length()));

        rapidjson::Value id;
        id.SetString(rapidjson::StringRef(this->id.c_str(), this->id.length()));

        rapidjson::Value status;
        status.SetInt(this->status);

        rapidjson::Value type;
        type.SetInt(this->type);

        rapidjson::Value body;
        body.SetString(rapidjson::StringRef(this->body.c_str(), this->body.length()));


        d.AddMember("From", from, allocator);
        d.AddMember("ID", id, allocator);
        d.AddMember("Status", status, allocator);
        d.AddMember("Type", type, allocator);
        d.AddMember("Body", body, allocator);
        
        rapidjson::StringBuffer buffer;
        rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
        d.Accept(writer);
        
        return buffer.GetString();
        
    }

It works normal but I noticed for the log, sometime this function took around 1 sec to finish. After check the timing for each function call, the line that took 1 sec is the d.SetObject().

What happened here? Is there any explanation and how to fix this?