“...Aw, I’m running 10.10.5 Yosemite and have been reluctant to upgrade. Heard Hi Sierra can really slow down an older machine....”
I’m running 10.13.2 High Sierra on a mid-2010 iMac w/32gb of ram, and it hasn’t slowed this old beast down a bit. Of course, I’m not compiling mountains of raw computer code...just email, web browsing and real-time streaming market data.
Mine’s about a yr newer than yours. Might try it. Just wish there was a way to revert if needed.