I had to take my late 2015 iMac into the Apple Store for an issue and they upgraded it to the current OS (High Sierra). I am wondering how is High Sierra performing on the late '15 iMac? Any firsthand reports? Is it slower? I was previously on El Capitan and I am concerned if things will run slower/worse now. Is that possible? The reason I am asking is I have had some bad luck with updating Apple products in the past. Thanks!