Why is non-blocking socket connect so slow?

when I do 100 non-block socket connection in 1 thread,it is very slow(the number of connection increased one by one),but if I do a blocking socket connection in 100 parallel threads(one connect per thread), it is very fast(get done immediately )

sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (fcntl(sock, F_SETFL,O_NONBLOCK)!=0)
{
 perror("fcntl nonblock");
 return -1;
}

if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR,&reuseAddr, sizeof(reuseAddr))!=0)
{
  perror("reuse addr");
  return -1;
}

sAddr.sin_addr.s_addr = inet_addr(SRV_ADDR);
sAddr.sin_port = htons(1972);

if ((n=connect(sock, (const struct sockaddr *) &sAddr, sizeof(sAddr))) < 0) 
{
  if (errno !=EINPROGRESS) {
      perror("client connect error");
      return -1;
  }     
}
else if (n>=0)
{
  printf("#%d connected\n",sock);
}

return sock;