I was having lag issues the first couple days, so I started saying with the settings. I unlocked developer settings and changed animations to .5 and tried that for a day or two, but it just made things worse: the software started having glitches and dropped frames so everything looked janky. Eventually I ended up changing it back and the phone got better. I also changed the screen resolution back and forth between FHD and QHD, and I eventually settled in FHD. I have great eyesight and normally can tell the difference between the sharpness of displays, but honestly I haven't missed QHD and like some others have mentioned there seems to be a correlation between FHD and smoother performance. One app that seemed to have performance problems was Facebook: every time I clicked on an "instant article" it would struggle loading and drop frames, resulting in janky performance.
I was really down on the S8 for a few days because of these problems, but ever since then the phone seems to have adjusted and figured things out. It's running uch smoother now and noticably faster as well. In fsct, this is probably the fastest Samsung I've ever owned, and by a large margin. And I'm one of those people that are overly sensitive to lag and jitters, which is why Samsung phones have always disappointed me. But for now the S8 seems to have recovered and sorted itself out.