like i mentioned before, since Nougat ,most 3rd party apps that gives CPU info for some reason cant read the 2 BigCores so it just gives a sleep or stopped status.
but why use a 3rd party when you can enabled the cpu usage options in developer mode, ur better off with this information as its given to u in real time.
now if u say your phone is still very very slow even with 4 cores active, then ur phone has some issue, in anycsae ur last option is to return to factory settings with ur current rom and have a fresh start and check again. ur apps could just be slowing down the phone