(http://curl.haxx.se/bug/view.cgi?id=1603712) (known bug #36) --limit-rate
  (CURLOPT_MAX_SEND_SPEED_LARGE and CURLOPT_MAX_RECV_SPEED_LARGE) are broken
  on Windows (since 7.16.0, but that's when they were introduced as previous
  to that the limiting logic was made in the application only and not in the
  library). It was actually also broken on select()-based systems (as apposed
  to poll()) but we haven't had any such reports. We now use select(), Sleep()
  or delay() properly to sleep a while without waiting for anything input or
  output when the rate limiting is activated with the easy interface.