There's just no way that installing 1 game would noticeably slow down your computer unless there was an existing problem. Maybe the game installed some anti-cheat garbage that runs in the background at startup. Maybe your AV was a bit too overzealous with on-access scanning. It's impossible to say for sure here, but there's more at fault than just the OS. It's also worth noting you're talking about an OS now 4 versions old.