try to shut down the Loop with uWebSockets v.0.17.4 (see https://github.com/uNetworking/uWebSockets/issues/809#issuecomment-619246807)

shutdown.0.17.4-kqueue-fast.cpp

#include "App.h"

using namespace std::chrono_literals;
using namespace std::chrono;

#include "../uSockets/src/internal/eventing/epoll_kqueue.h"

/* Note that uWS::SSLApp({options}) is the same as uWS::App() when compiled without SSL support */

int main() {

    us_listen_socket_t * listenSocket = nullptr;
    auto before = steady_clock::now();

	/* Overly simple hello world app */
	uWS::SSLApp({
	  .key_file_name = "../misc/key.pem",
	  .cert_file_name = "../misc/cert.pem",
	  .passphrase = "1234"
	}).get("/*", [&listenSocket, &before](auto *res, auto *req) {
        res->end("hi there");
        std::cout << req->getUrl() << std::endl;
        if (req->getUrl()[1] == 'q') {
            auto loop = reinterpret_cast<us_loop_t*>(uWS::Loop::get());
            std::cout << "will shutdown " << loop << std::endl;
            std::thread shutdown([&listenSocket, &before, loop] {
                std::cout << "yes, will shutdown" << std::endl;
                std::this_thread::sleep_for(200ms);
                std::cout << "shutting down " << listenSocket << " " << loop->num_polls << std::endl;
                before = steady_clock::now();
                loop->num_polls = 0;
                us_wakeup_loop(loop);
            });
            shutdown.detach();
        }
    }).listen(3000, [&listenSocket](auto *token) {
	    if (token) {
		    std::cout << "token " << token << std::endl;
		    std::cout << "Listening on port " << 3000 << std::endl;
	    }
	    listenSocket = token;
	}).run();

    std::cout << "finished in " << duration_cast<milliseconds>(steady_clock::now() - before).count() << "ms" << std::endl;
}

shutdown.0.17.4-slow.cpp

#include "App.h"

using namespace std::chrono_literals;
using namespace std::chrono;

/* Note that uWS::SSLApp({options}) is the same as uWS::App() when compiled without SSL support */

int main() {

    us_listen_socket_t * listenSocket = nullptr;
    auto before = steady_clock::now();

	/* Overly simple hello world app */
	uWS::SSLApp({
	  .key_file_name = "../misc/key.pem",
	  .cert_file_name = "../misc/cert.pem",
	  .passphrase = "1234"
	}).get("/*", [&listenSocket, &before](auto *res, auto *req) {
        res->end("hi there");
        std::cout << req->getUrl() << std::endl;
        if (req->getUrl()[1] == 'q') {
            std::cout << "will shutdown" << std::endl;
            std::thread shutdown([&listenSocket, &before] {
                std::cout << "yes, will shutdown" << std::endl;
                std::this_thread::sleep_for(200ms);
                std::cout << "shutting down " << listenSocket << std::endl;
                before = steady_clock::now();
                us_listen_socket_close(1, listenSocket);
                std::cout << "closed in " << duration_cast<milliseconds>(steady_clock::now() - before).count() << "ms" << std::endl;
            });
            shutdown.detach();
        }
    }).listen(3000, [&listenSocket](auto *token) {
	    if (token) {
		    std::cout << "token " << token << std::endl;
		    std::cout << "Listening on port " << 3000 << std::endl;
	    }
	    listenSocket = token;
	}).run();

    std::cout << "finished in " << duration_cast<milliseconds>(steady_clock::now() - before).count() << "ms" << std::endl;
}