iMac (Late 2015) using swap when there's still plenty of RAM left
This is all anecdotal, tested using a sample size of 2, so barely counts as experiment...

Restart the Mac, it's been up 12 days. 

That would be fine for a machine with a lot of RAM, but it's going to hit a low RAM machine harder.
I'd say the simple solution would be to double the RAM.
Two of the machines here are always on, the one with only 10GB RAM used to slow down after only a few days [it now has 26GB &amp; no longer does that], the one with 64GB really never slows down.

I think the used swap figure only gets reset at reboot, btw. I'm not sure it's necessarily an indicator of current activity [but I'm certainly open to correction on that]