Machine slow after HDD replacement with Samgung 840 EVO SSD

After the installation the machine suddenly got really slow. In the Activity Monitor, the kernel_task is eating 330% of the cpu, and the graphics show that the system is loading up to 87% of the cpu. Already tried to boot with the original HDD, booting with the original HDD and the Samsung EVO attached to an USB3 to SATA adapter but the machine always behaves the same way. Also noticed that when booting, in the first seconds the framerate of the cursor is normal but few seconds later it already start to drop. The Apple Hardware Test gives me an SMC error (PFM006) but my guess is that it is related to the thermal sensor in the HDD which is not present anymore. Tried to reset NVRAM and SMC but still no success. Took the machine to an experienced technician to check if there's something wrong with the power supply. Will report on any news.

The iMac had a SMC problem before the upgrade. Machine will be reverted to original factory state and we will try the warranty.