While Apple stated that the new MacOS High Sierra 10.xx will run on older Mac's (including laptops, iMac, and of course the monster Mac Pro), this latest OS upgrade does consume more resources that the previous Sierra and much more than the predecessor El Capitan. Launch the Activity Monitor utility and take a look. I believe Apple said High Sierra would run on computers as old as 2009. HOWEVER, any Mac older than say, 2012 will be challenged - particularly when attempting to run the hoggish SL viewers.
I was fortunate and fiscally able to max out my current late 2013 IMac with the best upgrades available at the time, including the biggest and fastest graphics card, the latest and fastest i7 dual quad core processors, and a sweet pure SSD *hard drive. Also loaded up with 16 GB's of memory. Only the most recent iMac's are now catching up with mine.
Bottom line, I have seen some minor glitches with my iMac and running the latest FS viewer with MacOS High Sierra. Tweaks in Preferences have helped a lot. I am resisting clearing the disc cache and/or re-installing FS viewer as yet. I do need to clean out some old inventory and perhaps clear inventory and texture caches. since I have over 200GB's of free space on my SSD, I maxed out the disc cache size, and that seemed to help, too.
Finally, try running with your ethernet option versus on WiFi. That should give a boost - especially on the older Mac's and particularly if you have an older WiFi router or Cable /modem router. A hard wired connection is always better. For now, my setup is running fine. Well, except for a couple FPS 'Falling Snow' lag ;-) Hope this post is informative and helpful.