The SSL/TLS process is a heavy one, it involves algorithm negotiation between client and server, key exchanges, cyphering, decyphering and authentication. But what’s surprising is, that the server you’re connecting to can directly influence the performance of your client and its CPU consumption.
I had a php command line process spawning child processes and connecting through SSL to a web server, in 2 scenarios. The first scenario was to an out of the box Apache httpd server with mod_ssl, and the second scenario was to an out of the box Nginx with the SSL module. Both were using the exact same box, and were “out of the box” meaning I used the default configuration for both.
In the first scenario I was able to spawn no more than 6 (!) php processes before the box running them began to show load, and the CPU queue started to fill up. Each php child was taking between 15%-30% cpu at any given moment.
In the second scenario, I was able to spawn 40 (!!) php child processes without the box being loaded. Each php child was taking around 1.5% cpu.
I’m no SSL expert, and there might be a way to configure Apache to inflict less load on the connecting client. There is also SSLSessionCache which might relieve load from both the server and the client. But the “out of the box” configuration shows that Nginx is a real winner again.
If you can, avoid SSL altogether. If not, terminate it at a front-end before proceeding to Apache.