High server load with Curl (and usleep)

Question

I'm facing very high load averages when performing a high number of curl multi requests.

Basically we have a platform that for each visit it will 20-40 requests to remote feeds (low latency, response time is less than 800ms). I've tested every part of the platform and the only one that seems to create the bottleneck is curl.

Our code is as follows:

foreach ($connomains as $i => $url)
    {
        $conn[$i] = curl_init($url);
        curl_setopt($conn[$i], CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($conn[$i], CURLOPT_SSL_VERIFYPEER, 0);
        curl_setopt($conn[$i], CURLOPT_SSL_VERIFYHOST, 0);
        curl_setopt($conn[$i], CURLOPT_FORBID_REUSE,0);
        curl_setopt($conn[$i], CURLOPT_FRESH_CONNECT, 0);
        curl_setopt($conn[$i], CURLOPT_INTERFACE,$interface);

        global $countrycode;
        curl_setopt($conn[$i], CURLOPT_TIMEOUT_MS, 1*500 ); 
        curl_setopt($conn[$i], CURLOPT_CONNECTTIMEOUT_MS, 500 );
        curl_setopt($conn[$i], CURLOPT_NOSIGNAL, 1); 
        curl_multi_add_handle ($mh,$conn[$i]);
    }

    do
    {
        $mrc = curl_multi_exec($mh, $active);
        //usleep(1);  // I've tried with a usleep here as well, no difference
    }while ($mrc == CURLM_CALL_MULTI_PERFORM);

    while ($active and $mrc == CURLM_OK)
    {
        if (curl_multi_select($mh, $active) != -1)
        {
            do
            {
                $mrc = curl_multi_exec($mh, $active);
            } while ($mrc == CURLM_CALL_MULTI_PERFORM);
        }
        else
        {
            usleep(10); // This doesn't seem to make any difference either.
        }       
    }

Our Curl configuration: (built with c-ares)

curl-config --features --protocols SSL IPv6 UnixSockets AsynchDNS NTLM NTLM_WB HTTPS-proxy DICT FILE FTP FTPS GOPHER HTTP HTTPS IMAP IMAPS POP3 POP3S SMB SMBS SMTP SMTPS TELNET TFTP

So basically on our stress tests, what happens after sending a certain amount of traffic, the load average goes beyond 100 and curl starts taking more than 10 seconds to reply (with a 500ms timeout). I've tried everything that came to my mind already. Any ideas here?

Thanks in advance.