It's not a bad idea to retain a non-subscription copy of the software available on your computer.
As well, you don't know whether disc capacity usage is the cause of the computer running more slowly. As the hardware is considerably older than the latest operating system, one should not rule out the possibility that the two are improperly matched, causing a performance hit. You could test for this by rolling back your OS to, say, El Capitan (10.11.6) and see whether computer performance improves.