For the first time ever I saw frame rates below 10 on ESO.
Oddly, it coincided with Keep raids by a specific enemy alliance. Not sure if they are related.
As someone who thought it might be PC based, I was wrong, because it is not due to lack of GPU/PC. Closing the client completely, and relogging resolved it, for a while.
My Rig:
Intel i7-4960X
RoG Rampage IV Black
GeForce GTX 780 Ti
BenQ XL2720T