I am running Plasma 5.8.3 with kernel 4.8.6 on my Arch Linux box. I am using SDDM as my login manager and my Nvidia GPU has for some time now been running at 60 to 62 degrees celsius in the desktop. I found out if I killed the sddm.service that the gpu temp would fall back to low 40 degrees celsius. My issue now is to figure out is this caused by sddm or nvidia driver. My card is a 970 GTX.
Fixed this by disabling and then reenabling the sddm.service with systemctl for some reason. Very strange but my gpu is back to 42 degrees celsius.