I play my (current release) fighting games on a self-built PC on a 4K TV in my living room, and on a 5K iMac running Steam on Windows ( via Bootcamp. Were you on a hard drive at that time? If so, maybe your drive was on the way out. I’ve had plenty of nightmares with malware/viruses/stupidme causing the bootcamp partition to get hosed and eventually re-formatted, but never slow.
However, I am still puzzled at how a system installed on a separate drive partition made your MacOS usage slow. As an avid Mac user since 1995, and a former Apple employee… I really feel your pain.
