CPU throttled to 0.8GHz due to "GPU LIMIT PROCHOT"
I'm posting this three years late, but I still wanted to offer my experience, perhaps to people still scratching their heads because of this issue.

What I've experienced is not entirely identical to the OP's case, namely the machine of concern is a MacBook instead of an iMac. However, the symptoms were highly similar: the system was not under much load, sensors read normal temperatures, yet the CPU throttles down to 0.8 GHz, slowing everything to a crawl. Resetting SMC and NVRAM does nothing, hardware test reveals nothing. I searched all over the internet for possible reasons but I wasn't able solve the problem.

The solution, at least for my case, was ridiculously simple. One day I decided to pop the back cover of my MacBook Pro open, and after a solid round of dust cleaning, the issue magically disappeared. So perhaps, try doing a routine cleaning, it might help. I'm not guaranteeing anything, but I do hope this might save a small portion of people some time.