Description:
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After investigating the reasons for the high CPU load generated by an application sending many curl requests with 20 parallel PHP-cli scripts, I've found out the main reason is the PROXY option set to the curl handle.

The following code is generating a 0.5-1.5% CPU load at the moment of curl_exec when run without the proxy and 12-25% when using any HTTP proxy (I've tried many different ones). It is NOT happening with a SOCK5 proxy type.

With many threads running many requests at the same time, it soon becomes critical.

I'm using:

curl 7.55.1 (x86_64-conda_cos6-linux-gnu) libcurl/7.55.1 OpenSSL/1.0.2l zlib/1.2.11 libssh2/1.8.0
Release-Date: 2017-08-14
Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP UnixSockets HTTPS-proxy

Test script:
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do {
$ch = curl_init();
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
curl_setopt($ch, CURLOPT_TIMEOUT, 10);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HEADER, false);
curl_setopt($ch, CURLOPT_ENCODING, 'identity');
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($ch, CURLOPT_PROXY, 'xxx');
curl_setopt($ch, CURLOPT_PROXYPORT, 'xxx');
curl_setopt($ch, CURLOPT_PROXYTYPE, CURLPROXY_HTTP);
//curl_setopt($ch, CURLOPT_PROXYTYPE, CURLPROXY_SOCKS5);
curl_setopt($ch, CURLOPT_URL, 'https://api.ipify.org?format=json');
curl_exec($ch);
curl_close($ch);
sleep(10);
} while (true);