What is 'symptomsd" and why is it at times using 60% of my iMac's CPU?
I have an iMac and had the same issue. It appears to be a bug in Mac OS X. 

In my case, I had both Ethernet and Wifi turned on for my iMac. Turning Off Ethernet in System Preferences &gt; Network, keeping Wifi as the only connection, prevents the issue completely. Now, the symptomsd consumes about 0.2 ~ 0.3 % CPU load as it's supposed to. 

I started to experience only recently after upgrade to Mac OS X 10.11.1. I guess this is a bug introduced recently.