Macbook Pro at 88% system CPU at all times

From one day to the next, out of nowhere I have super sluggish performance, where even the cursor barely moves. Even starting up takes forever. I managed to launch Activity Monitor and the system CPU is at 88% at all times. Rebooting ram didn't help. Going into safe mode made it even worse. Did Disk Utility tests, everything is fine. Don't know what else to do. Running El Capitan, 16 GB Ram, bought the machine December of 2012 and never had any issues until now. What else can I do? What could it be? No software was installed, nothing changed from one day to the next .