Debug network performance with minimal Winsock2 application

I have a very basic Winsock2 TCP client - complete list below - that just blows up a bunch of bytes. However, it is very slow over the network; the data just leaks out.

Here's what I tried and found (both Windows PCs are on the same local network):

• Launching this app from one computer to another is slow - it takes ~ 50 seconds to send 8MB.
• Two different servers - netcat and a specially written one (as simple as the client below) gave the same results.
• taskmgr shows how CPU and network are mostly used.
• Running this app with a server on one computer is fast - it takes ~ 1-2 seconds to send 8MB.
• Another client, netcat, works just fine - it takes ~ 7s to send 20MB of data. (I used the nc that ships with Cygwin.)
• Changing the buffer size (1 * 4096, 16 * 4096, and 128 * 4096) didn't make a big difference.
• Running pretty much the same code on Linux boxes on a different LAN worked fine.
• Adding a few print statements around the call send
  
  shows that we spend most of our time blocking.
• On the server side, we see a bunch of receiving blocks <= 4K (no matter what size of buffers the sender is sending). However, this also happens with other clients such as netcat, which runs at full speed.

Any ideas? Thanks in advance for any advice.

#include <winsock2.h>
#include <iostream>

using namespace std;

enum { bytecount = 8388608 };
enum { bufsz = 16*4096 };

int main(int argc, TCHAR* argv[])
{
  WSADATA wsaData;
  WSAStartup(MAKEWORD(2,2), &wsaData);

  struct sockaddr_in sa;
  memset(&sa, 0, sizeof sa);
  sa.sin_family = AF_INET;
  sa.sin_port = htons(9898);
  sa.sin_addr.s_addr = inet_addr("157.54.144.70");
  if (sa.sin_addr.s_addr == -1) {
    cerr << "inet_addr: " << WSAGetLastError() << endl;
    return 1;
  }

  char *blob = new char[bufsz];
  for (int i = 0; i < bufsz; ++i) blob[i] = (char) i;

  SOCKET s = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);
  if (s == INVALID_SOCKET) {
    cerr << "socket: " << WSAGetLastError() << endl;
    return 1;
  }

  int res = connect(s, reinterpret_cast<sockaddr*>(&sa), sizeof sa);
  if (res != 0) {
    cerr << "connect: " << WSAGetLastError() << endl;
    return 1;
  }

  int sent;
  for (int j = 0; j < bytecount; j += sent) {
    sent = send(s, blob, bufsz, 0);
    if (sent < 0) {
      cerr << "send: " << WSAGetLastError() << endl;
      return 1;
    }
  }

  closesocket(s);

  return 0;
}