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 > 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.

symptomsd runs as part of the CrashReportor framework.

I have the same issue with symptomsd process eating cpu (mouse lags and so on), when my mac connected to the web with LAN (thunderbolt 2 lan dongle) and Transmission working full force.

I guess i've found a fix for this, dunno if it works for you - goto network settings, choose your lan connection, goto advanced, choose hardware tab, then configure manually, change your duplex to "full-duplex, flow-control", other things leave as it is (mine is "speed-1000baseT"/ "mtu-standard 1500" / AVB enabled).

Hope this helps.