I originally replied to this thread back in early October, right after my initial "upgrade" to High Sierra (done at the behest of Apple Support in order to "fix" another problem I was having with iCloud on my iPhone). As a result of the installation of High Sierra, I was getting lots of freezing on my iMac requiring rebooting. After figuring out that the freezing might be related to a video issue, and ceasing usage of the QuickTime player application (as well as no longer leaving webpages open to YouTube videos), the instances of outright freezing diminished significantly. Now, it only happens once every few days.
However, I've been observing that I'm still getting short lags in the video updating when moving windows around the screen, or scrolling through documents. These lags appear to be somewhere in the few-hundred millisecond range ... long enough to observe, and create choppy window movement, but apparently short enough to not generally result in a full-on freeze up of the system. It is highly repeatable, and happens any time a window is moved, or scrolled. It also seems to affect text entry, and other functions requiring UI updates. I'm not sure that this is simply a video problem, or if it is something else going on at the system level that is robbing the UI of enough CPU cycles to be able to update for short periods of time (less than a second, but enough to observe UI "stuttering". I'm seeing it occasionally lag (while typing characters) even now while typing this into the Reply box on this webpage (in Safari). I can move the mouse part way across the screen and have it "jump" several inches, rather than follow drag motion.
I've tried a different mouse, just be make sure I wasn't losing mouse updates. No difference, even with a different mouse.
Its certainly better than outright freezing requiring reboots, but still pretty very annoying that High Sierra has degraded the performance of what used to be a fine machine.
I've installed every update since the original update to High Sierra, but none seem to have yet cured the problem. Still waiting for an update that actually fixes this...