Chosen Solution
When using iMac (Late 2013 27" which has been fitted with an SSD) Kernel _Task throttles CPU. Instead of the CPU running at 3.2GHz, it will run no faster than 0.8GHz and fans run at 3000+rpm. Have checked all temperature sensors and all work fine. The mac is also not overheating. When running Apple hardware diagnostics, it tells me there may be a problem with the system management controller (SMC). Resetting the SMC temporarily fixes the kernel_task problem but it always returns. Any ideas what may be causing this issue?
Why are you using such an old SSD? This is what you should be using Samsung 860 EVO and you need this in-line sensor OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later
