Multithreading and parallel processes with c++

I would like to create a c++ webserver that will perform a task for each user that lands on my website. Since the task might be computationally heavy (for now just a long sleep), I'd like to handle each user on a different thread. I'm using mongoose to set up a webserver.

The different processes (in my code below just one, aka server1) are set up correctly and seem to function correctly. However, the threads seem to be queuing one after the other so if 2 users hit the end point, the second user must wait until the first user finishes. What am I missing? Do the threads run out of scope? Is there a "thread-manager" that I should be using?

#include "../../mongoose.h"
#include <unistd.h>
#include <iostream>
#include <stdlib.h>
#include <thread>


//what happens whenever someone lands on an endpoint
void myEvent(struct mg_connection *conn){
    //long delay...
    std::thread mythread(usleep, 2*5000000);
    mythread.join();
    mg_send_header(conn, "Content-Type", "text/plain");
    mg_printf_data(conn, "This is a reply from server instance # %s",
    (char *) conn->server_param);
}

static int ev_handler(struct mg_connection *conn, enum mg_event ev) {
    if (ev == MG_REQUEST) {
        myEvent(conn);
        return MG_TRUE;
    } else if (ev == MG_AUTH) {
        return MG_TRUE;
    } else {
        return MG_FALSE; 
    }
}

static void *serve(void *server) {
    for (;;) mg_poll_server((struct mg_server *) server, 1000);
    return NULL;
}

int main(void) {
    struct mg_server *server1;
    server1 = mg_create_server((void *) "1", ev_handler);
    mg_set_option(server1, "listening_port", "8080");
    mg_start_thread(serve, server1);
    getchar();
    return 0;
}