High system CPU usage

I have a mid-2012 MacBook Pro (10.12.6, 2.5GHz intel core i5, 16GB RAM, 500GB SSD) which despite its age had been running remarkably well - starting up/shutting down within 10 seconds, running cpu intensive software etc.

After not using it for a couple of months I’ve booted it up and it is now unusably slow. Activity monitor is reporting 80-85% system CPU usage even with no apps open.

What could be causing this?