Mysterious 1 second wait for requests that exceed backlog

#include<iostream>
# include <cstring>
# include <chrono>
# include <sstream>
# include <iomanip>
# include <arpa/inet.h>
# include<sys/socket.h>
# include <unistd.h>
using namespace std;
const string&appname(const string&s=string()){
    static string_appname;
    if(!s.empty())_appname=s;
    return_appname;
}
using namespace std::chrono;
US>string timestr() {
    system_clock::time_point scp=system_clock::now();
    time_tt=system_clock::to_time_t(scp);
    tmlt;
    US>stringstream s;
    ss<put_time(localtime_r(&t,&lt), "%Y/%m/%e%H:%M:%S")<", "<setw(3)<setfill('0')<duration_cast<millisecond_(:glock_time)))
    returns.str();
}
void log(ostream&out, const string&s) {out<<timestr()<"<appname()<":"<s<endl;}
void log(const string&s) {log(cout,s);}
void log_error(const string&s) {log(cerr,s+":"+strror(errno)));}
#define LOG_ERROR()log_error(string(__FILE__)+":"+to_string(__LINE__))
US>structure Sock {
    Sock(intsock):_sock{sock}{}
    ~Sock() { if(_sock>=0)::close(_sock);}
    operator int() {return_sock;}
    void close() { if(_sock>=0)::close(_sock);_sock=-1;}
private:
    int_sock;
    Sock(){}Sock(const Sock&){}Sock&operator=(const Sock&){return*this;}
};
# define IF_ERROR_RETURN(cond) if(cond) {LOG_ERROR(); return;}
int client(const int PORT=8000, const size_t N=0x100000){
    auto func=[](auto id, int port){
        US>//#string idhead {to_string(id)+":"};
        auto log_with_id = [&] (autos) {log(idhead+s);};
        Socksock=socket(AF_INET, SOCK_STREAM, 0);
        IF_ERROR_RETURN(sock<0);
        structure sockaddr_inaddr;
        addr.sin_family=AF_INET;
        addr.sin_port = htons(port);
        IF_ERROR_RETURN(inet_pton(AF_INET, "127.0.0.1", &addr.sin_addr)<=0);
        IF_ERROR_RETURN(connect(sock, (struct sockaddr*)&addr,sizeof(addr))<0);
        log_with_id("connection established");
        structure sockaddr_in local_address;
        socklen_taddr_size=sizeof(local_address);
        IF_ERROR_RETURN (getsockname(sock, (sockaddr*) (void*)&local_address,&addr_size)!=0);
        log_with_id("localport:"+to_string(ntohs(local_address.sin_port)));
        shutdown (sock, SHUT_WR);
        log_with_id("write shutdown");
        shutdown (sock, SHUT_RD);
        log_with_id("recv shutdown");
        sock.close();
    };
    for(inti=0;i<N;++i){
        strings {to_string(i)};
        log(s+":started");
        func(i,PORT);
        log(s+":finished");
    }
    return 0;
}
#define IF_ERROR_BREAK(cond) if(cond) {LOG_ERROR();break;}
void server(const int PORT=8000) {
    auto log_server=[](const strings) { log(string("server:")+s);};
    structure sockaddr_inaddr;
    structure sockaddr_inclient;
    intsock;
    Sock serverSock=socket(AF_INET, SOCK_STREAM, 0);
    IF_ERROR_RETURN (serverSock<0);
    addr.sin_family=AF_INET;
    addr.sin_port = htons(PORT);
    addr.sin_addr.s_addr=INADDR_ANY;
    IF_ERROR_RETURN(bind(serverSock, (struct sockaddr*)&addr,sizeof(addr))<0);
    IF_ERROR_RETURN (listen(serverSock, 5)<0);
    while(true){
        socklen_tlen=sizeof(client);
        Socksock=accept(serverSock, (struct sockaddr*)&client,&len);
        IF_ERROR_BREAK(sock<0);
        log_server("remoteport:"+to_string(ntohs(client.sin_port)));
        char buff [4096];
        ssize_treadlen=read(sock, buff, sizeof(buff)));
        IF_ERROR_BREAK(readlen<0);
        if(readlen!=0)log_server("read!!!not empty!!!");
        sock.close();
        log_server("closed");
    }
    serverSock.close();
}
int main(intargc, char*argv[], char*envp[]){
    appname(argv[0]);
    if(argc>1&string(argv[1])=="--server"){server();return0;}
    else {return client();}
}

trap'pkill-P$$'INT
./maxbacklog_time --server&
server=$!
./maxbacklog_time
kill$server

...
October 27, 2022 18:44:31,859./maxbacklog_time:server:remoteport:35178
...
2022/10/27 18:44:31,859./maxbacklog_time:221:localport:35200
...
2022/10/27 18:44:31,865./maxbacklog_time:222:localport:35206
...
2022/10/27 18:44:31,865./maxbacklog_time:223:localport:35222
...
2022/10/27 18:44:31,865./maxbacklog_time:224:localport:35232
...
October 27, 2022 18:44:31,865./maxbacklog_time:224:finished
2022/10/27 18:44:31,865./maxbacklog_time:225:started
2022/10/27 18:44:31,867./maxbacklog_time:server:closed
October 27, 2022 18:44:31,867./maxbacklog_time:server:remoteport:35186
2022/10/27 18:44:31,867./maxbacklog_time:server:closed
October 27, 2022 18:44:31,867./maxbacklog_time:server:remoteport:35196
2022/10/27 18:44:31,867./maxbacklog_time:server:closed
October 27, 2022 18:44:31,867./maxbacklog_time:server:remoteport:35200
2022/10/27 18:44:31,867./maxbacklog_time:server:closed
October 27, 2022 18:44:31,867./maxbacklog_time:server:remoteport:35206
2022/10/27 18:44:31,867./maxbacklog_time:server:closed
October 27, 2022 18:44:31,867./maxbacklog_time:server:remoteport:35222
2022/10/27 18:44:31,867./maxbacklog_time:server:closed
October 27, 2022 18:44:31,867./maxbacklog_time:server:remoteport:35232
2022/10/27 18:44:31,867./maxbacklog_time:server:closed
2022/10/27 18:44:32,879./maxbacklog_time:225:connection established
...