C++ performance degradation (or code must be improved ?)

I am developping a simple program that copies the same string into another one in a loop. I use Visual Studio C++ 2019 Community Edition, and the project type is "Command line".

If I run it for 3,42 seconds then the calculated number of copies per second is 130 601 397, but if I run it for 77,97 seconds then the number of copies per second is 47 469 296...

The more time the program is running, the more performance degradation there is...

Here is the code :

#include <iostream>
#include <cstdlib>
#include <signal.h>
#include <chrono>
#include <string>

using namespace std;

unsigned long repeats_counter = 0;

std::chrono::steady_clock::time_point t1;
std::chrono::steady_clock::time_point t2;

// When CTRL+C (SIGINT), this is executed 
signal_callback_handler(int signum) {
    t2 = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double> diff = t2 - t1;
    std::cout << "Total execution time " << diff.count() << " s\n";
    unsigned long average_repeats_per_sec = (unsigned long)(repeats_counter / diff.count());
    std::cout << "Number of average repeats per second was " << 
    std::to_string(average_repeats_per_sec) << "\n";
    std::cout << "Number of average repeats per minute was " << 
    std::to_string(average_repeats_per_sec * 60) << "\n";
    cout << "Number of effective repeats = " << repeats_counter << endl;
    // Terminate program
    exit(signum);
}


int main()
{
    signal(SIGINT, signal_callback_handler);
    signal(SIGTERM, signal_callback_handler);

    std::string from_str, to_str;

    cout << "Start copying. CTRL+C to stop." << endl;
    t1 = std::chrono::high_resolution_clock::now();
    from_str = "the string to be copied";
    while (true) {
        to_str = from_str;
        repeats_counter++;
    }
    return 0;
}